The southern margin of Junggar Basin was characterized by a large contain of gypsum mudstone and mudstone which had a high dispersion and expansibility. Its high formation pressure required the use of high density drilling fluid with a narrow drilling fluid equivalent density window which was favorable to sticking and well leakage. In this investigation，a high density leak proof oil based drilling fluid was obtained by optimizing various ingredients as well as composite leak proof agents. The temperature，electrolytes and drilling cuttings tolerance of oil based drilling fluid was assessed and the sealing performance of formulated leak proof oil based drilling fluid was studied. The results showed that the formulated oil based drilling fluid had good temperature resistance and salt tolerance. Under the condition of 180℃ and 2.5 g/cm³ density，the rheology and emulsifying capacity of drilling fluid were stable. The emulsion-breaking voltage was bigger than 1000 V. Additionally，its rheological property and emulsifying capacity were not affected when exposed to 15% CaSO₄，12% NaCl，20% CaCl₂ and 10% drilling cuttings. When a composite plugging agent，including 1% bitumen FD-F，1% calcium carbonate（600 meshes）and 0.5% graphite powder，and 3% oil-soluble leak proof plugging agent（LCM）as the components of oil based drilling fluid，it had good plugging and leak proof performances.The sealing pressure of high density leak proof oil based drilling fluid was 7.75 MPa and the leak proof pressure at a width of 1—5 mm was 7.0 MPa.