In power system electromagnetic transient calculation, because of various kinds of sudden changes, there will be numerical oscillation in numerical calculation. In order to avoid the numerical oscillation in power system electromagnetic transient calculation effectively, the 2-stage 3-order diagonally implicit Runge-Kutta method is proposed for electromagnetic transient calculation. Through theoretic analysis, the proposed numerical integration method is of nonlinear B-stability, which also means that the method is of energy dissipation characteristic or nonlinear damping characteristic. From example results, this paper confirms that the proposed method can be free of numerical oscillation effectively though sudden change happens. Besides, compared with the present methods, using the proposed method to implement electromagnetic transient calculation will not add any extra computation load. This work is supported by National Natural Science Foundation of China (No. 51377098).