Abstract: The theory of new-democratic society is a new development of Marxism in China. It was the only social design in 50s last century which might lead China to industrialization and modernization. After new China was founded, the theory was largely put into practice. But, Mao Zedong suddenly abandoned the theory and stopped practicing the theory while the value of the theory was demonstrated. Why did Mao Zedong do so? The actual cause is that Mao Zedong fell into theoretical area of errors while considering the socialist future of China.