Materialistic dialectics is the philosophical premise to understand the change of the main social contradiction in the new era. From the perspective of materialist dialectics, the change of the main social contradiction in the report of the nineteenth congress of CCP is a historical conclusion of the interaction between the subjective dialectics and the objective dialectics, which is the connotation of materialist dialectics and the organic unity of the symbol of the new era, the richness of the connotation and the innovation of the working methods. In the dimension of era value , the change of main social contradiction marks the periodical qualitative change of the development of socialism with Chinese characteristics, the transformation of the strategic goal of China’s socialist modernization and the change of the international status of socialism with Chinese characteristics towards the center of the world stage. In the dimension of ideological connotation, the change of the main contradiction in the new era is mainly reflected in the objective change of the contradictory conditions, the synchronic change of the contradictory factors, and the fundamental change of the contradictory relations. In the dimension of working method, the change of the main social contradiction brings new problems to work practice, such as new concerns, setting up dialectical thinking and innovating work spirit.