Population equilibrium development is the valuable guide and ideal pursuit of China's population policy and has significant implications for regional equilibrium development. However, there are still misunderstandings about interpreting population spatial equilibrium, and its scientific connotation requires further explanation and promotion. Population spatial equilibrium gives the meaning of space based on population equilibrium, which can be understood as the spatial equilibrium of population distribution. When other factors remain unchanged, the net migration between regions is zero; that is, the total number of immigrants of each region are equal to its total number of emigrants to achieve the dynamic equilibrium of population distribution among regions. A migration stream will exist between regions in an equilibrium state. Migration and equilibrium can coexist, but the population distribution between regions remains dynamic and stable, and the population spatial distribution structure will not change. Specific conditions cause the inherent stipulation of population spatial equilibrium. The equilibrium point will also change with economic, social, resource, and environmental changes. Equilibrium is a relative concept derived from the instantaneous concept; instantaneous equilibrium is an ideal state in the long term. The population spatial distribution is constantly changing from disequilibrium to equilibrium and from equilibrium to disequilibrium. This paper describes a theoretical model of multi-regional population spatial equilibrium based on the regional wage rate, human climate suitability index, urban crowding degree, and distance. Using the Yangtze River Delta as an example, the study analyzes the equilibrium population spatial distribution under current economic, social, environmental, and population conditions. The study found that the population of the study area is concentrated along the river and the coast, mainly in the south of Jiangsu, along the coast of Hangzhou Bay, and the canal from Nanjing to the north of Jiangsu. By comparing the equilibrium population with the current distribution, the Yangtze River Delta can be divided into quasi-equilibrium, attractive, and repulsive areas. The attractive areas, such as Shanghai, Nanjing, and Hangzhou, have large population agglomeration potential. In contrast, the repulsive areas are mainly located in northern and western Anhui, northern Zhejiang, and other areas close to the quasi-equilibrium area. This suggests that under free migration, the population will further concentrate in the areas of Shanghai, Nanjing, Hangzhou, and southern Jiangsu.