There is plenty of non-biological CO2 in the north marginal basins of South China Sea. Based on patterns of gas origins and classified parameters (i.e.: δ13CCO2, 3He/4He) as well as specific geological conditions of gas migration and accumulation, two types of CO2 sources are classified into crustal-type CO2 or mixing of crust-mantle mixed type CO2 controlled by both muddy diapiric thermal fluid, and mantle-type CO2 acted by mantle-derived volcanic activity and deep fault. The crustal-type and mixing of crust-mantle type CO2 is widely distributed. The mantle-type CO2 accumulation is mainly controlled by mantle-derived volcanic activity and deep fault.