As part of the southeast Asian monsoon system, the South China Sea (SCS) is characterized by monsoon wind transition from northeasterly to southwesterly in the boreal spring. Correspondingly, a spring warm pool (SWP) with mixed layer temperature warmer than 29¿C is formed and peaked in May over the central SCS and then decays in June. Calculations of the heat budget showed that the surface heat flux and ocean dynamics have different roles in the formation and decay of the SWP. The surface heat flux is found to play a dominant role for the formation of the SWP in May. The onset of the southwest monsoon corresponds to an increase in cloudiness and rainfall and a strong wind speed in the SCS that decrease the surface heat flux in June. Moreover, the onset of the southwest monsoon also changes oceanic circulation pattern that advects cold water from the northwest to the southeast and pumps cold water into the upper mixed layer. These cooling effects overcome the warming effect of the surface heat flux, resulting in the decay of the SWP in June.