Abstract: The Triassic and deeper formations in Mahu sag in Junggar basin are of abnormal high pressure which is one of the important factor controlling the oil and gas accumulation. This paper studied the distribution, origin and preservation condition of the abnormal high pressure using measured formation pressure, interval transit time and seismic data, combined with the sedimentary and tectonic settings as well as well compaction curve and well pressure prediction and basin simulation techniques. The result shows that the top surface of overpressure appears in different formations in different areas in Mahu sag. Its Jurassic and newer strata may not have overpressure in northwestern Mahu sag, but the overpressures are found in the Jurassic of southeastern Mahu sag and Dabasong swell. The main formation mechanism of the abnormal high pressure is undercompaction due to rapid sedimentation, while the contribution from organic material maturing is not big. The major factor of the abnormal high pressure formation near the fault zones in margin of the sag attributes to the other source overpressure that may create from the fault activity. The residual pressure evolution since the Triassic can be divided into four stages: free residual pressure stage, slowly increasing stage of it, rapid increasing stage and slowly decreasing stage. It is the unique sedimentary and tectonic evolution of Mahu sag that forms the conditions of top, bottom and edge plates of the abnormally pressured fluid compartment, which allows the abnormal high pressure to be kept to present