In view of the vibration and plastic deformation of subgrade under high-speed train load, the 2.5D finite element method for elastoplastic subgrade was proposed. The subgrade deformation induced by high-speed train load was regarded as material nonlinearity, and the improved Mohr-Coulomb model was adopted to simulate the elastoplastic soil. Based on the 2.5D finite element method, the displacement calculated by elastic theory was regarded as tentative displacement. When the soil reached yielding according to the improved Mohr-Coulomb yield criterion, the tangent stiffness iteration method, backward Euler integration algorithm, and uniform tangent modulus algorithm were introduced into the algorithm to update the stiffness matrix, and the plastic deformation induced by high-speed train operation was solved iteratively. On this basis, a 2.5D finite element method for elastoplastic subgrade was established, where the orbit was treated as Eular beam, the modified multi-frequency train load was adopted to simulate train operation, and the viscoelstic artificial boundary was used to treat the truncated boundary of the finite element model. The closed-form solution and field measurement results were compared to verify the correctness and reliability of the proposed model. Results indicate that the model can be used to efficiently solve the vibration and accumulative deformation problems of ground induced by high-speed train load.