Exploitation of gas hydrate requires an improved understanding of the behavior of gassy soil. In this study, a numerical simulation technique is proposed that uses the discrete element method (DEM) to evaluate the undrained mechanical behavior of gassy sand, which consists of three phases—sand particles, water, and gas. The governing equation of gassy sand under undrained condition is derived and incorporated into the DEM simulation, in which gas solution and exsolution are considered. Undrained isotropic unloading and undrained triaxial compression tests on gassy sand samples with dissolved methane and carbon dioxide are simulated. Reasonable consistency is found when the simulation results are compared with experimental observation and theoretical calculation. The simulation highlights the significant influence of the gas solubility on the undrained behavior of gassy sand. The micro-responses of the observed phenomenon are also investigated.