Abstract: By use of the changes of acoustic logging data with buried depth and equivalent depth method, formation and evolution characteristics of overpressure of K₁n¹ mudstone in the north of Songliao Basin was studied. It was considered that the overpressure of K₁n¹ mudstone was universally developed in the north of Songliao Basin at present. The area of high value of overpressure distributes in Qijia-Gulong depression, and the maximum can reach up to more than 9 MPa now. Overpressure of K₁n¹ mudstone mainly formed during K₂m² sedimentary period and then increased gradually. At the end of K_{2m and E sedimentary period, the overpressure had ever decreased because of the activity of T1 faults. According to the formation and evolution process of overpressure and hydrocarbon generation-expulsion histories of source rocks, it was considered that ①the main hydrocarbon expulsion phase of K1n¹ source rock was at the end of E sedimentary period; ②the longest distance of migration downward of oil and gas from K1n¹ source rock was about 190 m. In most parts of Daqing Placanticline, Sanzhao depression, Qijia-Gulong depression, Longhupao terrace and Mingshui terrace, etc, oil and gas from K1n¹ source rocks could migrate downward into underlying Saertu oil layers. Only in local areas the oil and gas could migrate downward into Putaohua oil layers. ③Formation period of overpressure was earlier than the main expulsion period, which was favorable for sealing oil and gas. ④Overpressure of K1n¹ mudstone underwent ever depressure at the end of K2m and E sedimentary period after its formation, but its seal ability continuously increased until now. These conditions should be significant for the accumulation and distribution of oil and gas in Saertu, Putaohua oil layers in the north of Songliao Basin.}