During the hydrate mining process, the decomposition of gas hydrate products the deformation of sediments due to the accumulation of pore pressure, which usually contributes to geological disasters. This paper establishes a theoretical model of reservoir deformation evolution during the hydrate decomposition process. The model combines the phase equilibrium equation and the unsaturated thermos-poromechanical constitutive law, considering the significant capillary effect of fine-grained hydrate sediments. Through this model, the evolution of reservoir deformation during thermal recovery under undrained conditions are analyzed. The result shows the significant tensile strain occurs at the high level of pressure due to the hydrate decomposition, and this phenomenon is attributed to the density difference between the fluid and hydrate. Also, after the completed thermal stimulation, the deformation first decreases and then increases as the initial hydrate saturation increased. It is worth noting that the deformation is tensile but even at lower initial saturations. Thus, it is necessary to pay attention to the tensile failure of the reservoir.