Abstract: To scientifically evaluate the spatiotemporal evolution laws of agricultural land suitability (ALS) in the upper reaches of the Yangtze River, this study constructed a comprehensive evaluation model that integrated geographic information system (GIS), entropy weight method, and dominant factor identification method. This study conducted a spatiotemporal analysis of ALS in the study area from 2005 to 2020. Results showed that: (1) ALS exhibited significant spatial differentiation. Sichuan Basin area (SCB) had favorable natural conditions and was an ideal area for agricultural production. Spatial distribution of Western Sichuan and eastern Tibet areas (WS-ET) was similar to that of Yunnan Plateau area (YNP). The concentrated and contiguous area of moderately and generally suitable level constituted a relatively favorable region for agricultural production. 60% of Guizhou Plateau area (GZP) belonged to the relatively suitable area, but it had obvious patchy characteristics and was a potential area for agricultural production and development. The rest of the agricultural zoning demonstrated a small territorial space area, mostly being at the general suitable level or below, which was rather unfavorable for agricultural production. (2) The agricultural land suitability area (ALSA) continued to decrease. From 2005 to 2010, the reduced area was concentrated in the unsuitable level of YNP and WS-ET and the suitable level of SCB and Southern region of Qiangtang (QT). From 2010 to 2015, the reduced area was concentrated in the relatively suitable level of YNP and SCB and the generally suitable level of WS-ET and SCB. The reduced area from 2015 to 2020 was concentrated in the relatively suitable level areas of WS-ET and GZP. (3) The centroid of overall suitability and all the suitability levels showed differentiated shift trajectories. Shift trajectory of agricultural land centroids was the result of joint driving effect of optimization and concentration of cultivated land and climate change. (4) Soil erosion (SEro), annual precipitation (AP), and soil texture (STex) were the dominant factors of ALS, and the dominant regions were QT, SCB and GZP, WS-ET and YNP, respectively. The results showed that the spatiotemporal evolution characteristics and decreasing trend of agricultural land suitability area (ALSA) in the study area were significant. It was necessary to implement zonal regulation strategies based on the dominant factors to optimize the utilization of cultivated land resources and ensure regional food security.