Historical materialism is one of Marx’s greatest discoveries, which is of great significance in the history of human social thought. In Engels’ later years, there was a new trend in the development of capitalist society, where letters became an important carrier for Engels to carry out theoretical research, propaganda and struggle. Engels’ letters in his later years paid full attention to the reaction to the superstructure of the economic foundation and the relative independence of ideology. In addition, it emphasizes that the theory should keep pace with the times, also puts forward and expounds the thought of “historical resultant force theory.” Furthermore, we firmly opposed dogmatism, contrarily adhered and developed scientific Marxism. Engels’ letters in his later years not only enriched and developed Marxist historical materialism, but also provided a strong theoretical basis for the great practice of socialism with Chinese characteristics in the new era.