Several fundament problems like the mathematical model and numerical simulation of gas production from natural gas hydrate reservoir, similarity principle of gas hydrate physical simulation and exploitation method of gas hydrate reservoir are researched. A mathematical model is developed to simulate the hydrate dissociation in hydrate-bearing porous medium. The model can be used to analyze the effects of the flow of multiphase fluids, the intrinsic kinetic process of hydrate dissociation, the endothermic process of hydrate dissociation, ice-water phase equilibrium, the variation of permeability, the convection and conduction on the hydrate dissociation and gas and water production. For a single gas hydrate reservoir exploited only by depressurization, the study shows that water freezing appears in the formation with the dissociation of gas hydrate under certain condition due to energy deficiency for hydrate dissociation, which can lead to blockage of flow path and stopping of gas production. For a hydrate reservoir underlain by a free gas zone exploited by depressurization, the results show that the overlying hydrate zone can supply an amount of gas to improve the output of production well and evidently prolong the lifespan of gas reservoir. The combination method of warn water flooding with depressurization is proposed, which can comprehensively utilize the advantage of thermal stimulation and depressurization. The method is characterized by a long stable gas production period with a high gas rate.