Abstract: The special existence of gas in gassy sand makes its basic properties different from those of general unsaturated soils. To analyze the undrained mechanical properties and static liquefaction instability of gassy sand, this work extends the triaxial test system equipped with dual pressure chamber to achieve the preparation and triaxial tests on the gassy sand with high degree of saturation. Based on the modified triaxial test system, a series of triaxal tests on the saturated sand and gassy sand are conducted, and the second-order work criterion and determinant criterion for identifying static liquefaction of the gassy sand are proposed. The results show that a small amount of entrapped gas can significantly increase the shear strength and axial deformation requested for triggering static liquefaction, and thus improve its resistance to static liquefaction. The second-order work criterion implicitly includes the current strain path, which can accurately determine the triggering of static liquefaction of the gassy sand, while the determinant criterion is a necessary condition for the second-order work criterion, determining whether there is a strain path that makes the soils unstable under the current state, which is the lower limit of static liquefaction instability.