Abstract: Based on the data of the fifth, sixth, and seventh national population censuses, this paper analyzes the evolution characteristics and influencing factors of the spatial distribution of population in Southwest China from 2000 to 2020 at county level. The main findings are as follows. Since 2000, the proportion of population on both sides of the Hu Line in Southwest China has remained stable on the whole, but the trend of population distribution on both sides is completely opposite. The population distribution on the southeast side of the line tends to be centralized, while that on the northwest side tends to be balanced. The regional differentiation characteristics of population increase and decrease are obvious. In recent years, most regions in Sichuan, Chongqing and Guizhou have achieved a stop and return increase in population, but Yunnan Province has fallen into a comprehensive population contraction. Population changes generally show a trend of 'small concentration, large contraction', and the provincial or municipal governments have become the core of population growth. The regression results show that the administrative level is the key to determine the population change in Southwest China. The higher the administrative level, the stronger the population attraction. In addition, the linear distance between each county and city unit in the region and the provincial or municipal government residence, that is, the location conditions, also determine the future population development trend of the county and city unit to a certain extent.