Formation and evolution characteristics of KlqnI mudstone overpressure in North Songliao Basin are quantitatively studied by acoustic logging data. It is considered that the overpressure is universally developed in North Songliao Basin at present; The high-value area of the overpressure distributes in the center of Qijia-Gulong and Sanzhao sags; the maximum value can reach more than 12 MPa. The overpressure was mainly formed in Kin4 sedimentary period and then increased gradually ; because of the activity of T2 faults, the overpressure released and decreased in the end of Kin, KEm and E sedimentary period. By means of the researches on the formation and evolution process of the overpressure, four following petroleum significances are obtained : ①the main hydrocarbon expulsion phase of K1 qnI source rock was the end of K2m sedimentary period; ②the longest distance of the migration downward of oil and gas from Klqn1 source rock is up to about 350 m. In most parts of Daqing Plaeanticline,Sanzhao and Qijia-Gulong Sags ere, the oil and gas could migrate downward into Fuyu oil layer. The area that oil and gas migrated downward into Yangdachengzi oil layer was smaller than that into Fuyu oil layer; ③Formation period of the overpressure was earlier than the main hydrocarbon expulsion period, that was favorable ior sealing the oil and gas; ④After the overpressure formation for K1qn mudstone, although tile overpressure ever released and decreased in the period stated above, its seal ability has still gradually increased up to now.