The theoretical study of China's semi-feudal and semi-colonial economic formation is the most outstanding contribution of Wang Yanan's life. He combined the basic principles of Marxism with the reality of China, and made a profound analysis of the semi-feudal and semi-colonial economic form of old China. He has published a series of influential monographs, including China's Economic Theory, as well as two other representative works, An Overview of the Feudal System of Landlord Economy in China and A Study of the Bureaucracy in China. Through the above works, I have my own unique and incisive views on the commodity economy theory, capital theory, profit and interest theory, economic crisis theory and bureaucratic political theory of old China, and formed a complete economic theoretical system. In these systematic studies, he emphasized that there were foreign capital, bureaucratic comprador capital, national capital and feudal economy in the social and economic relations of modern old China, which was transitional. At that time, in the long-term debate on the social and economic nature of modern old China, he made a scientific and systematic theoretical exposition and advocated the correct view that modern old China was a semi-feudal and semi-colonial society. This is of great significance for understanding the nature, object and strategic strategy of China's new-democratic revolution, and it is an outstanding contribution to the theory of new-democratic revolution. In the academic discussion on the stages of China's ancient history. Wang Yanan is a famous scholar who studies "feudalism in the Western Zhou Dynasty".