In 2018, the precipitation in the first rainy season in South China (Guangdong Province and Guangxi Zhuang Autonomous Region) was abnormally little, with the regional average total precipitation of 410.9mm, which was about 40% less than the average climate.The results of this study show that the precipitation negative anomaly in the first rainy season in South China in 2018 is related to the warm sea temperature in the tropical northern Pacific (TNP). The SST positive anomaly in the tropical North Pacific resulted in the cyclonic circulation anomaly in the Northwest Pacific and the east wind anomaly in the South China Sea through Mastuno-Gill response ,which weakened the South China Sea summer monsoon. Abnormal anticyclonic circulation existed in the lower troposphere from South China to South China Sea. There is an abnormal transport of water vapor from the mid-latitude North Pacific Ocean to the tropical Pacific Ocean via Kuroshio Sea Area-South China Sea-South China Sea-Philippine Islands, and South China is an abnormal divergence region of water vapor. On the other hand, the warm sea temperature anomaly in TNP region has caused abnormal convergence in the lower layer, abnormal divergence in the upper layer, and abnormal upward movement in the region, leading abnormal convergence in the upper layer, abnormal divergence in the lower layer, and abnormal subsidence movement in South China.This circulation arrangement is not conducive to the production of precipitation in South China, resulting in unusually little precipitation in South China's first rainy season in 2018. The physical mechanism of TNP regional warm sea temperature anomaly causing negative precipitation anomaly in South China's first rainy season in 2018 is also verified by the average test results of 30 member-sets of ECHAM5 model.