Since the 1920s，Japanese socialists have pursued the establishment of a unified proletarian political party. However，Communists and Social Democrats fought fiercely around the unity of proletarian political parties until they were completely dissolved in 1940. After World War II，the Allied forces occupied Japan and carried out democratic reform，which created an important historical opportunity for the development of Japan’s socialist movement. In this context，Japanese socialists took action one after another to rebuild the pre war political parties. Therefore，they once again face the problem of the unity and division of the proletarian political party. In the early post-war period from August to September 1945，Social Democrats and Communists had the opportunity to jointly establish a unified political party. However，in that process，influenced by the historical development process in the early stage of the occupation，the historical contradiction between the Japanese Communist Party and the Social Democrats，the unity of the three left，middle and right factions of social Nationalism after the war，and the strong political belief of the Communists，the Communists and Social Democrats finally decided to separate and build a new party. Therefore，the Japanese Socialist Movement was divided into two systems：the Socialist Party and the Communist Party，which has had profound impact on Japan’s Socialist movement in the postwar era.