Abstract: It is significant to study the source rock maturity history in the C sag of the Weixinan depression for the deep water exploration in this area of Beibuwan basin.Based on geohistory and thermal history of the C sag,the source rock maturity history of the sag is reconstructed with the usage of Easy%Ro model.The studies indicate:(1) In the early rifting stage,the heat flow in the C sag of the Weixinan depression reached the maximum(77 mW/m²), then decreased gradually and is about 54 mW/m² at present. (2) The Liushagang Formation source rock in the C sag began to generate hydrocarbon (Ro=0.5%) at about 51 Ma,reached the hydrocarbon generation peak(Ro=1%) at about 42 Ma, and the high maturity (Ro=1.3%) at about 17 Ma. (3)Comparatively,maturity of the Liushagang Formation source rock of Well Wei1, in the deep water exploration area is higher and the hydrocarbon generation capacity is stronger,so the deep water exploration area has more cheerful prospect.