Typhoon "Kai-Tak" (2012) had caused extreme precipitation along coastal areas in Guangdong and Guangxi when the intensity of the South China Sea summer monsoon reached its strongest stage. In order to figure out the causes of extreme precipitation associated with typhoon "Kai-Tak", we simulated the track, intensity, rain belt position, and precipitation intensity of typhoon "Kai-Tak" after the summer monsoon intensity reduced. Results showed that southwest monsoon had been weakened in the South China Sea; typhoon intensity was weakened; typhoon precipitation was reduced among the typhoon range as a whole; and typhoon rain band moved southward. Thus, the weakened South China Sea summer monsoon was not in favor of producing extreme precipitation. During the precipitation intensity simulation, we found that after monsoon intensity was reduced, the typhoon precipitation intensity was not weakened obviously around its peak point but obviously weakened in other period. In order to determine the reasons, the hourly evolution characteristics of the typhoon precipitation and the typhoon track simulation were analyzed. It is found that control simulation experiment has the difference in the landing time of typhoon with the sensitivity simulation experiment, which basically corresponds to the time difference between the hourly variation peak of typhoon precipitation. This indicates that other complicated factors in the process of typhoon landing led to a non-obvious decrease in precipitation caused by typhoon landing after the weakening of the monsoon, while beyond the process of typhoon landing, the weakened summer monsoon has a significant decreasing effect on typhoon precipitation.