<p>The South China Sea summer monsoon (SCSSM) plays a critical role to the advance and strength of East Asian summer rainfall, and it is of great importance to assess the performance of the General Circulation Models (GCMs) participating in Coupled Model Intercomparison Project Phase 6 (CMIP6) in simulating the SCSSM onset. Compared to observations, most CMIP6 GCMs simulate the onset of the SCSSM 1 to 7 pentads later, with only CanESM5 showing an earlier onset by 1 pentad. The multi-model ensemble mean demonstrates a delay of 3 pentads. The delay in the onset of the SCSSM can be attributed to two main factors. Firstly, 18 out of 20 GCMs exhibit a cold air temperature bias in the upper-middle troposphere over southern China during April and May. This bias reduces the land-sea thermal contrast between southern China and the South China Sea, contributing to a later onset of the SCSSM. Notably, the stronger the cold air temperature bias, the longer the delay in onset observed among these models. Secondly, a widespread cold bias in the sea surface temperature (SST) of the South China Sea, which persists from April to May, suppresses convection aloft, further delaying the onset of the SCSSM. The two factors mentioned are largely independent, and their binary linear combination accounts for over one-third of the variance in the onset date bias of the SCSSM in these GCMs. The standardized regression coefficients for two factors are −0.35 and −0.34, indicating that both factors contribute similarly to the SCSSM onset date bias in the CMIP6 GCMs. These results are useful for improving the simulation and future projection of the East Asian summer climate.</p>