The denudation thickness of Paleogene—Neogene unconformity in the Biyang Sag has been calculated with the Milankovitch cycle method. It has been concluded that in the center of the sag, only Liaozhuang Formation is denuded. In Anpeng area which locates in the southeast of the sag, the denudation thickness is less than 100 m and increases gradually to the west, northwest and northern margin of the sag, and finally exceeds 650 m around the missing boundary of Liaozhuang Formation. Hetaoyuan Formation is also denuded near to the sag boundary, mainly 4.1~666.5 m thick, and the total denudation thickness is over 1 000 m. The known oilfields in the sag mainly locate in the area with denudation thickness larger than 200 m.