This research examines the origin and development of the communist propaganda machine in Guangzhou and Hong Kong from 1936 to the eve of the Cultural Revolution. Drawing on archives and personal accounts, I argue that the communist propaganda cadres in Guangzhou and Hong Kong formed an institutional network that influenced the ideological system in South China throughout the mid-twentieth century. Before 1949, Hong Kong leftist newspapers, news agency, schools and united front organization trained propaganda workers who played a major role in both Hong Kong and Guangzhou after 1949. In the early PRC, the majority of these workers moved to Guangzhou and helped cadres from North China to set up propaganda apparatuses quickly, and surprisingly managed to run newspapers with Hong Kong characteristics. Workers who stayed behind in Hong Kong developed a distinctly soft and practical propaganda style to win hearts and minds of local people and negotiate with colonial authority, but they were still deeply influenced by their former colleagues in Guangdong that built many previously-unnoticed institutions to control the leftist propaganda in Hong Kong after 1949. They also frequently sent staff and their children to Guangzhou for training and education, which in effect enhanced the link of personnel. This border-crossing connection strengthened the effect of communist propaganda in these two cities in the 1950s and even influenced cultural events in later periods, showing the power of this institutionalized cross-region communist ideological system.