Abstract: As one of the main methods to solve the partial differential equations, Green function method is the key teaching content of mathematical physics equation. Green function has been widely used in solving Poisson equation and heat conduction equation. Since Green function satisfies the specific problem of definite solution and has the property of symmetry, it is always difficult to master the main content of Green function in teaching. In this paper, the mechanical meaning of Green function and its symmetry are elaborated by combining the bending problem of beam and the reciprocal theorem of work. Compared with the traditional teaching of Green function, it is simpler and easier to understand to explain with the bending problem of beam.