Abstract: In the textbook of elasticity mechanics, the stress field of gravity dam with triangular cross-section is given by taking cubic polynomial stress function. However, the cross-section of engineering gravity dam is generally trapezoid, and its dam crest is not triangular cusp. Combining the semi-inverse method of stress function with the stress function of the tiny body, the stress function applicable to trapezoidal gravity dam is found out. Based on the boundary conditions of trapezoidal gravity dam force and using the Saint-Venant principle, the analytical formula of the stress field of trapezoidal gravity dam is given. Numerical simulation results of the stress field of trapezoidal gravity dam are given by finite element calculation. The analytical formula of the stress field is in good agreement with the finite element simulation result, which shows the correctness of the analytical formula of the stress field of trapezoidal gravity dam. The analytical formula of the stress field of trapezoidal gravity dam has important theoretical guiding significance and application value for structural strength and design of a gravity dam in hydraulic engineering.