The structure of photovoltaic power plant changes from time to time, which leads to nonlinear, time-invariant charateristics of the mathematical model of the photovoltaic power plant. The automatic generation control system based on PID control algorithm with constant parameters can’t cope with them well. To improve the performance of the automatic generation control system, its control algorithm is studied. For photovoltaic plants, a discrete adaptive control algorithm is designed based on Popov superstability theory. In a 50 MW photovoltaic plant, the perfomances of the automatic generation control system based on this adaptive control algorithm and that based on PID algorithm with constant parameters are compared. The simulation proved the superiority and feasibility of the automatic generation control system based on this adaptive algorithm.