For lower voltages and fluctuated inputs of a standalone Photovoltaic (PV) inverter, a disturbance-rejection system with a double-closed-loop control was proposed. On the basis of the working principle of the system, the main parts of the inverter were modeled and a overall mathematical model was obtained. The inverter with an input voltage of 24 V, an output voltage of 311 V, a frequency of 50 Hz and an output power of 6 kW was illustrated as an example. In the design, the pole assignment method was used to calculate the controller parameters and the relation between fluctuation input and steady state error was described quantitatively. The simulation result indicates that the system with good stability and low steady state error can reject the disturbance. Its voltage change is 0.03%, total Harmonic Distortion(THD) is 1.03%, and the highest single harmonic content is less than 3%, which meets the requirements as well. The model and design method with theoretical guidance meaning is universal and can support the applications and design of high power inverters in other special fields of PV generation.