With the steady integration of renewable generation and electrical vehicles into power systems，the dynamics of the power system under random factors is receiving ever-increasing concern.In order to discuss the dynamic responses of the power system under random excitation，a nonlinear stochastic differential equation（SDE）model for one machine and infinite bus（OMIB）system is built，where the power fluctuation of renewable energy and electrical vehicles are treated as Gaussian white noise.Then the Heun numerical method is applied to the SDE to calculate the dynamics of rotor angle under random disturbance.The calculation step is firstly changed to study the numerical stability of the Heun method.The random excitations with different intensities are also applied to the SDE，and the impact of random excitation on the dynamics of rotor angle is analyzed.The effect of the time interval of random excitation on the rotor angle is also studied.Finally，according to the characteristics of the corresponding power spectrum，some conclusions of the impact of random excitation on the dynamics of the rotor angle are gained.