Abstract: To reveal the mechanical properties of the rocks of the Fengcheng Formation, the rocks of the Lower Permian Fengcheng Formation in the Mahu Sag of Junggar Basin were taken for the laboratory mechanical tests to obtain mechanical behaviors of the rocks, the influence of mineral compositions on strength parameters of the rocks of the Fengcheng Formation was then discussed. The rocks of the Fengcheng Formation in the Mahu Sag have strong heterogeneity, which results in obvious differences of mechanical properties of rocks with different lithologies. The mechanical strength and elastic modulus of dolomitic rocks are higher, but the Poisson's ratio is lower. The form of rock fracturing is relatively simple. Under uniaxial conditions, rock samples have strong brittleness, which is characterized by tensile fracturing, while under high confining pressure, rock samples have weaker brittleness and enhanced ductility, which is mainly characterized by single shear fracturing. The compressive strength, tensile strength and fracture toughness of rocks decrease with the increase of siliceous mineral content, while increase with the increase of calcium mineral content.