In the process of rapid urbanization, gentrification and commercialization have inevitably appeared in the historical districts of Jiangnan. Although they optimize the space environment of the historic district and promote the development of economy and tourism, they have resulted in disadvantages such as insufficient vitality of the space group, rigid space interface, lack of space quality and lack of regional culture. Mathematical calculation can be used to analyse the data of 15 historical blocks, determine the key points of the protection of historical blocks in Jiangnan area and then put forward reasonable construction strategies. This paper starts from the four elements of space group, space structure, space quality and space vitality, identifies the key points and key point of the historical block protection in Jiangnan area, and then puts forward the reasonable place building strategy.