Abstract: Gas trapped between the soil skeleton in quasi-saturated sand (gassy sand) takes on a special form, which gives it unique basic properties that differ from those of unsaturated soil. Currently, the undrained stress-strain response of gassy sand is insufficient and the critical state of gassy sand is not well-defined. For this purpose, the work modified a double walled triaxial testing system to achieve the preparation and test of gassy sand. Drained and undrained triaxial tests on saturated sand, as well as undrained triaxial tests on gassy sand, are conducted to analyze how entrapped gas affects the stress-strain response of gassy sand under undrained conditions. It was observed that the entrapped gas significantly increased the shear strength at the triggering of static liquefaction of gassy sand, augmented the axial strain required for initiating static liquefaction, and enhanced the ability of loose sand to resist static liquefaction. Besides, experimental evidence is provided to prove the consistency between the critical state characteristics of gassy sand and saturated sand. Moreover, this study provides a state-dependent model of gassy sand and demonstrates its effectiveness in capturing the mechanical behavior of gassy sand.