There were overpressures in south edge of the Junggar Basin, which could be proved by observed pressures, such as RFT and DST, and well log information, such as sonic defferential time and mud weights. The maximum value of pressure coefficient of the overpressure was 2.40 in the Well An6, and the distribution of the overpressure was controlled by the stratum, especially by the Anjihaihe Formation in age of Eogene. The overpressure was mainly in the second and third row of anticlines, and part of the Qigu anticline. The generation of overpressures might have a complicated cause, namely, a synthetic result of the rapid sedimentation in Cenozoic, the sealing of heavy mudstone, such as the Anjihaihe Formation, the percentage of mudstone in the formation being 85.2% to 99.7%, and the tectonic stresses. The role of the diagenic dehydration, hydrocarbon generation had relatively smaller effect for the overpressures.