The first stage was in 1930s and 1940s. During this period, his work focused on compiling the input-output table of the United States and establishing the theoretical system of input-output analysis. 1930 after he settled in the United States, he received two letters of appointment at the same time, one from the National Bureau of Economic Research of new york and the other from Harvard University. In order to meet the needs of research work, he chose the latter career. 1932, that is, in his second year of teaching at Harvard University, he absorbed the experience of the former Soviet Union in making economic balance tables from 1923 to 1924, and planned to design a statistical estimation table of commodity flows among various economic sectors in the United States. 1936, Leontief published his input-output model and outline of input-output theory, and published the article "Quantitative Relationship between Input and Output in American Economic System", which received wide attention. In the next 20 years, Leontief presided over and guided the compilation of the national economic input-output tables of the United States 19 19, 1929, 1939 and 194'7, which put forward a new direction for studying American economic problems. He also tested and confirmed his input-output theory in his compilation work. Within 20 years, Leontief was an associate professor of economics at Harvard University from 1939 to 1945, and was promoted to professor the following year. 1943 to 1945 served as Russian economic adviser to the US Strategic Office, and 194 1 to 1947 served as adviser to the US Department of Labor. In the second stage, from 1950s to 1960s, Leontief regarded input-output analysis as a comprehensive economic analysis tool. Therefore, after solving the problem of compiling the input-output table of a country's national economy, he further explored the method of using this method to deeply study different parts or individual links. These issues include: production structure, dynamic analysis, cost and price, planning and forecasting, regional economic relations, foreign trade, economic development of underdeveloped countries and the impact of disarmament on production and employment. Leontief's input-output analysis table reflects his achievements in these aspects. In 1950s, Leontief and Chenery co-authored "Research on American Economic Structure: Exploration of Input-output Analysis in Theoretical and Empirical Materials" in 1953, which opened up a wider application field for input-output theory. In the 1960s, the capitalist social and economic crisis intensified, and the Secretary-General of the United Nations invited him as an economic adviser. In this way, he worked in the United Nations for two years from 196 1 to 1962. During this period, he investigated the possibility of his theory going global. 196 1 was hired by the us department of labor and served as a consultant for the second time. 1965, just left the Labor Department and was hired as a consultant by the US Department of Commerce. From 65438 to 0966, Leontief sorted out his theoretical system and published the book Input-Output Economics. This book is an important summary. In the same year, he published Essays on Economics: Theory and Its Formation. 1967 was hired as a French industrial journalist. In the third stage, after entering the 1970s, Leontief's theory developed further. According to the statistics of the United Nations from 65438 to 0979, this theory has been widely adopted by 89 countries and regions in the world. From 65438 to 0987, Leontief visited China with former US President Nixon. He praised China's economic system with planned economy as the mainstay and market regulation as the supplement. After returning to China, he published a commentary on "Socialism Works in China" and spoke highly of the economic recovery and development since the founding of New China. This commentary caused a sensation in the western world and played a positive role in the western countries' understanding of China.

In 1950s and 1960s, Leontief skillfully applied the input-output method to many disciplines of economics, and achieved success one after another. Input-output method has been recognized by the society, and many academic institutions, government departments and scholars have begun to use input-output method to compile input-output tables. From 65438 to 0974, the United Nations commissioned Leontief to establish a global input model to study the possible changes in the world economy in the last 20 years of this century and the solutions that the international community can adopt. The future of the world economy is one of the achievements of Leontief's research.