In the process of learning structural mechanics, drawing bending moment diagram is the basic issue, which belongs to key examination part and starting point of the structural internal force analysis. The bending moment diagram of statically indeterminate structures is complicated, which seriously affects students' understanding and mastery of basic contents. This study aims to find out the general law about bending moment diagram of symmetrical rigid frame by comparing the bending moment diagram of stationary symmetric rigid frame and analyzing the characteristics of bending moment diagram of statically indeterminate symmetric rigid frame under anti-symmetrical load.