In order to improve the frequency stability of solar-thermal microgrid, this paper puts forward a method by combination of optimization toolbox control and optimization function based on Nonlinear Control Design (NCD) to improve PID control, and applies it to the power system frequency regulation. Mathematical models of photovoltaic systems and thermal power are respectively established, and the total amount of change in the active role of photovoltaic systems and load is applied to the thermal power generating units for reducing the deviation of the power system frequency caused by the instability of PV system, by means of the NCD controller. Based on the solar-thermal microgrid model on Simulink, the simulation and data analysis have verified the effectiveness of the proposed scheme. Compared with the conventional frequency regulation, the proposed scheme achieves faster system response, smaller overshoot and stronger robustness, and reduces the disturbance of the power system active balance caused by the photovoltaic grid-connected. This work is supported by National Natural Science Foundation of China (No. 61304177) and Scientific Research Project of Sichuan Education Department (No. 15ZB0345).