Existentialism was put forward as an irrational trend of thought and has been active as the main school of modern western philosophy. Existentialism is mainly composed of theistic existentialism, atheistic existentialism and humanitarian existentialism. The concept of existentialism was first put forward by the famous philosopher Heidegger in his book Being and Time, in which Heidegger explained and applied existentialism as an independent title for the first time. Sartre studied under Heidegger and Husserl, and his philosophy was deeply influenced by Nietzsche. As a master of existentialism, Sartre not only inherited and developed Husserl's irrationalism, but also sublated Kierkegaard's religious mysticism, thus forming his unique philosophical thought and making indelible contributions to the continued development of existentialism. All kinds of ideas and viewpoints of existential philosophy are based on the concept of existence. After World War II, France was full of depression and decadence, and intellectuals formed a cynical fashion in endless hesitation, depression and loneliness. Sartre's existential philosophy, which preached the existence of life and absolute freedom, was highly sought after and became a hot topic of social research at that time, and then became the most popular philosophical trend in France and even Europe in the 1950s and 1960s. Throughout Sartre's existential philosophy, it is mainly composed of three parts: "existence precedes essence", "free choice" and "responsibility" First, the existence of human existence precedes the essence, which is regarded as the starting point of existential philosophy by existential philosophers. They believe that to experience human existence, we must proceed from the actual living state of human beings, and human existence comes from human's free choice. What is existence? Sartre believes that "existence is anything that people can think of from alienation: it will suddenly drown you, it will always be above you, it will press on you like a huge, inactive beast-besides, there is nothing here" [1]. "Existence precedes essence" is the first principle of existentialism and the recognized starting point of existentialist philosophers. This is a special reference to human beings, that is, human existence precedes human essence, rather than other existence precedes other essence. The "existence" here refers to "human reality" or "human independence", not "existence" based on the basic problems of philosophy. Sartre pointed out that "the first thing is human existence, appearance and appearance, and then explain yourself." Specifically, on the one hand, actual existence precedes essence. As a subjective existence, what kind of person people will become in the future and how to develop are all decided by themselves, and human nature is the product of independent creation. Not simply following the appearance of "people", there will be "people"; On the other hand, people can give things the essence subjectively. Everything except "man" does not have the qualification of self-existence, so it is impossible to choose and create its own essence independently. Sartre believes that the problem of existence, as a basic problem of philosophy, has its practical significance. Sartre distinguished "existence" from "free existence" and "self-existence". The essence of some things beyond consciousness is "super-phenomenal", which is the unknown root and essence of people and things. The deep "existence" is "comfortable existence". This existence is neither active nor passive, neither positive nor negative, neither possible nor inevitable, and it is impossible to really know. Although it can exist forever, it is accidental, not inevitable. It is this "existence" that is a disgusting world full of absurdity and heresy. "Free existence" refers to the existence beyond consciousness, so the relative "self-existence" should be people's subjective consciousness. It is a real existence, but it is full of freedom and uncertainty, and it is also a kind of nothingness, which makes "now" a denial of self. As the structure of consciousness, nothingness can never reach a complete state, always in a state of change, and never reach the eternity of freedom. Only when there is death can its essence be realized. Therefore, human existence is in a constantly changing situation. Logically, Sartre believes that "the existence of freedom" precedes "the existence of self-action", and there is no "self-action" without freedom; But "self-action" also gives "freedom" value and meaning. Without "self-action", "freedom" will become meaningless and an abstract existence with no value. So the real existence is "self-existence", because these two kinds of existence are opposite in essence, but they can achieve some unity. First, if "self-existence" is divorced from "self-existence", it will become abstract; Secondly, if "the existence of freedom" and "the existence of self" are separated, it will be worthless.