The APSIM-Maize model was calibrated and validated using crop and soil data from agro-meteorological experimental stations in maize planting zones in Southwest China (SWC). The spatiotemporal distribution characteristics in simulated rainfed yield of spring maize were analyzed to explore the impacts of changes in main climatic factors during the maize growing season on yields from 1961 to 2010. We found that the APSIM model performed well in simulating phenology, above-ground biomass and yield of six representative spring maize varieties. The root mean squared error between simulated and observed growing periods was less than 8 d for all varieties and the normalized root mean squared error between simulated and observed above-ground biomass and yield was less than 29% for four varieties. Growing season total solar radiation decreased in the central, southern and northern SWC;≥8℃ accumulated temperature increased evidently in western SWC; daily diurnal temperature range decreased in western and southeastern SWC; and total precipitation decreased in central SWC. As a result, the simulated rainfed yield of spring maize declined at 46% of all study stations in SWC, especially in central, eastern and southern SWC. The contribution of decreasing solar radiation, increasing temperature, decreasing precipitation and decreasing diurnal temperature range during maize growing season to yield reduction was 32%,40%,1% and -2%,respectively, in stations which had a significant decline in yield.