Abstract: Magmatism of the oil/gas area is one of the problems that is caused extensive attention by petroleum geologists. During oil/gas exploration in the East China Sea, especially in the middle-southerm part of the Xihu Sag, the Tertiary magmatism is considerably revealed. This will directly affect the deployment of oil/gas exploration. In this paper, the distribution of the Tertiary magmatic rock in time and space, the relationship of the magmatism with oil/gas accumulation and the trap type related with magmatic rock is analyzed. A result of the analysis Shows that the magmatism still be advantage to oil/gas accumulation in a certan degree as compared wieh the destroy effest,whereas the developmental area of magmatic rock could is regarded as the next frontier for oil/gas exploration in the middle-southern part of the Xihu Sag.