Abstract: Due to the density contrast between the hydrate and methane gas, the pore pressure is accumulated in the sediment during the decomposition process of methane hydrate. This accumulation of pore pressure decreases the magnitude of effective stress, further triggering potential geological disasters such as landslide. This paper establishes a theoretical framework to investigate the evolution of fluid pressure in the hydrate-bearing sediments during the decomposition process. This model consists of two parts: an unsaturated thermo-poromechanical constitutive law as well as a phase equilibrium equation. Compared with the existing studies, the present work incorporates the effect of pore volume change into the pressure built-up model. In addition, the capillary effect is considered, which plays a nontrivial role in fine-grained sediments. Based on this model, the evolution of fluid pressure is investigated in undrained conditions. It is shown that four mechanisms mainly contribute to the pressure built-up: the density contrast between decomposing hydrate and producing fluid, the variation of pore volume, the compaction of hydrate due to variation of capillary pressure, and the thermal deformation of pore constituents induced by temperature change. Among these mechanisms, the density contrast dominates the pore pressure accumulation. Under the combined effect of these contributions, the evolution of fluid pressure exhibits a strong nonlinearity during the decomposition process and can reach up to dozens of mega Pascal. Nevertheless, this high-level pressure built-up results in a significant tensile strain, yielding potential fracturing of the sediment.