The thermal regime of the Weibei Sag, Changwei Depression, was modeled by using the vitrinite reflectance(Ro)and apatite fission track data. The thermal gradient was up to 4.4～5.3℃/100m during the Eocene to Miocene(sedimentary period of the Kongdian Formation)from six wells’ modeling results. The thermal gradients decreased with the geological evolution of the sag. It was 4.2～4.9℃/100m, 4.0～4.7℃/100m during sedimentary periods of the 4th Member, Shahejie Formation and the Guantao Formation respectively. The present thermal gradient was 3.8～4.2℃/100m. At the same time, two hydrocarbon entrapments were identified by using the homogenization temperatures of fluid inclusions, combining with the burial history of samples, thermal history and hydrocarbon generation history of source rocks. The first hydrocarbon entrapment occurred during 52～50.5 Ma ago, and the second entrapment was during 43.5～36 Ma ago, the latter was the most important entrapment to the hydrocarbon accumulation in the Weibei Sag.