Abstract: South China Sea summer monsoon (SCSSM) is an important system to affect the rainfall over China, and the onset represents the development of summer circulations over East Asia. The evident circulation changes (including the western Pacific subtropical high shifting off from South China Sea, prevailing low-level southwesterly wind and high-level leaning east wind over South China Sea, etc.) occur from pre-onset to onset. Local variations are often adopted to define SCSSM indices, while a SCSSM index taking large-scale circulations into account can avoid some effects from synoptic-scale processes. Air-sea and air-land interactions are main factors impacting on the SCSSM onset, descriptions of which are defective in GCMs (global and regional). Moreover, artificial effects increase the uncertainty of GCM’s SCSSM simulation and forecast.