The present invention relates to the technical field of numerical simulation for marine natural gas hydrate resource exploitation, and particularly relates to a method for calculating a seepage velocity field in the hydrate sediment based on an unstructured grid finite element method. The method comprises: S1. according to geological data, establishing a geological model of the hydrate depressurized reservoir, including the inner boundary of the wellbore and the outer boundary of the reservoir; S2. establishing an unstructured grid system on the geological model; S3. establishing a multi-fieldcoupled mathematical model for hydrate depressurization, including a gas phase pressure equation, a water phase saturation equation, a hydrate decomposition equation, an initial condition, an inner boundary condition and an outer boundary condition; S4. on the established unstructured grid system, establishing an element stiffness equation by using the Galerkin finite element method, and solvingthe multi-field coupled mathematical model discretely to obtain the spatial distribution of the pressure field and the saturation field; and S5. according to the obtained pressure field and the saturation field on the grid node, calculating the velocity field of the gas phase and the water phase by using the element stiffness equation. According to the method provided by the present invention, thecalculation speed and accuracy are improved.