Based on the fifth and sixth census data, this paper takes 16 prefecture-level and above urban administrative units in Chengdu-Chongqing city cluster as the research object, and conducts comprehensive calculation and analysis of their shrinking spatial pattern, basic features and formation mechanism. The results show the following:1)13 out of the 16 cities of Chengdu-Chongqing city cluster are contracting, accounting for 81.25% of the total urban area, mainly concentrated in the central region of Chengdu-Chongqing city cluster. Three of them are in a non-shrinking state, accounting for 18.75% of the total urban population, and they mainly distribute on both sides of the urban agglomerations. The shrinkage of 14 municipal districts accounted for 29.79% of the total. The contraction phenomenon occurred in 64 city jurisdiction counties, accounting for 65.98 percent of the total number of cities jurisdiction counties. 2) The overall type of shrink city can be divided into three major types:global contraction, marginal contraction and local contraction. The global contraction includes 5 cities, accounting for about 38.46% of the total contraction cities, all located in the central part of Chengdu-Chongqing city cluster and showing continuous development trend. The marginal contraction mainly includes 4 cities, accounting for 30.77% of the total contraction. Local contraction mainly includes 4 cities, accounting for about 30.77% of the shrinking cities, mainly distributed in the central marginal region of Chengdu-Chongqing city cluster, non-shrinking municipal districts (counties) are surrounded by shrinking towns.3) The number of young children and working-age people has been decreasing, the number of the elderly population has been increasing and the burden of social support has been increasing; The population of education showed a trend of continuous increase, and the proportion of people with bachelor's degree or above increased significantly. The proportion of the employed population in the primary industry has declined dramatically, while that of the employed population in the secondary and tertiary industries has increased significantly, and the proportion of the employed population in the secondary industry has increased the most. 4) Based on the analysis of the formation mechanism of urban shrinkage, the central city's siphon effect, the lack of employment in the transformation of industrial structure, the increasing aging trend of population, natural factors and the appropriate interventions of the government constitute the key mechanism of urban contraction of Chengdu-Chongqing city cluster.