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The mechanical behaviour of gassy sand is rather complex owing to the inherent complex nature of sand and the occluded/dissolved gas. For better understanding of the behaviour of gassy sand under monotonic loading, a numerical model is presented in this paper. By considering the gas–water mixture in gassy sand as a homogenous pore fluid, the theory of two-phase saturated porous media is employed. Based on this theory, the impact of the occluded/dissolved gas is characterized by the relationship between the compressibility of pore fluid and the degree of saturation as well as the pore fluid pressure, derived from Boyle’s and Henry’s Laws. Then an advanced constitutive model for sand is combined with the porous media theory to capture the complex stress–strain behaviour of sand, using a single set of model parameters. Through the spatial and temporal discretization of the governing equations by the finite element method and the well-known θ-method, the porous model is numerically implemented as a user-defined element subroutine provided by ABAQUS, in which the implicit constitutive integration algorithm is used. Finally, the influences of gas type, degree of saturation, pore fluid pressure level, and physical state (void ratio and stress level) on the behaviour of gassy sand are studied by the numerical model. The model is validated by comparing the simulated results with laboratory test data from literature.