Abstract: The inverter new energy represented by the photovoltaic power supply has strong randomness and no moment of inertia, which can not be applied to the traditional generator characteristic model and the rotor motion equation in the electromechanical transient analysis method of the power system. This paper researches the randomness modeling method of photovoltaic power supply for algebraic calculation. In this paper, the unit random variable is Gauss white noise. The stochastic differential equation is established according to the structure of the photovoltaic power grid-connected device, and then the analytical solution of the equation is derived. Then, the modeling method, which considers multiple random factors, is analyzed. After that, the numerical solution of the equation is calculated by Maple software and compared with the waveform of the simulation circuit on the Matlab/Simulink platform; the results verify the effectiveness of the modeling method proposed in this paper. Finally, the application of this model in the electromechanical transient analysis of the new power system is researched. The sufficient condition critical value of the system stability is obtained by the Example, which breaks through the electromechanical transient calculation bottleneck caused by the new energy's non-rotational inertia.