Differential calculus studies the derivative and differential of function and its application in function research. The analytical methods used in the establishment of differential calculus have had a far-reaching impact on the development of the whole mathematics, and have been applied to many branches of mathematics, infiltrating into many fields such as natural science and technical science. The function of differential calculus is to show not only the state but also the process (movement) by mathematics in natural science. The basic idea of differential calculus is to consider whether the function can be expressed arbitrarily by linear function or polynomial function in a small range. Intuitively, for a function that can be expressed by a linear function at will, any tiny line segment on its graph is similar to a straight line. On such a curve, any point has a unique straight line-the "tangent" of that point. At this point, it is in a fairly small range and cannot be distinguished from the curve. This approximation simplifies the local study of complex functions. Differential calculus is based on a series of basic concepts such as real number, function, limit and continuity.