Abstract: Engels, as one of the founders of Marxist theory, he and Marx chose the scientific communist road through two different ways in their youth. However, Engels’ early thoughts have important implications for the creation of some of his later works. Among them, since studying the realistic object of the city is the realistic basis for the transformation of Engels’ thoughts, there are three important stages from the transformation of Engels’ thoughts, namely his attention to the situation of the workers in the Wupei Valley, the analysis of the causes of the situation of the British working class and the explanation of the relationship between private ownership and urban development. Looking at the theoretical connotation of Engels' transition from democracy to communism from the three stages will help people understand and establish a materialist dialectical view and a historical view of materialism.