Analysis of climatological observation temperature data reveals that during the southwesterly monsoon, there exists a low temperature zone east of Vietnam. It cools down from June to August and warms up in September. Meanwhile, during this period, the cold water mass spreads eastward to the deep basin. Numerical experiments validate the results of data analysis and further verify that there are two basic factors that induce the Vietnam cold water. The major one is the strong local positive wind stress curl, which leads to the divergence of sea surface water and the upward supplement of lower layer water in the deep basin. Another minor one is the alongshore component of southwesterly monsoon, which drives the offshore Ekman transport and coastal upwelling in the shallow region along the Vietnam coast. In addition, the negative wind stress curl in the southern South China Sea inputs negative vorticity to the ocean and drives a strong anticyclonic gyre, which affects the spatial distribution of the cold water evidently. Analysis of climatological observation temperature data reveals that during the southwesterly monsoon, there exists a low temperature zone east of Vietnam. It cools down from June to August and warms up in September. Meanwhile, during this period, the cold water mass spreads eastward to the deep basin. Numerical experiments validate the results of data analysis and further verify that there are two basic factors that induce the Vietnam cold water. The major one is the strong local positive wind stress curl, which leads to the divergence of sea surface water and the upward supplement of lower layer water in the deep basin. Another minor one is the alongshore component of southwesterly monsoon, which drives the offshore Ekman transport and coastal upwelling in the shallow region along the Vietnam coast. In addition, the negative wind stress curl in the southern South China Sea inputs negative vorticity to the ocean and drives a strong anticyclonic gyre, which affects the spatial distribu tion of the cold water evidently