Looking for renewable and non-polluting energy sources is the most important thing in today"s society. Solar is the best energy, how to effectively use solar energy has become the focus of the society. This paper proposes a three-level off-grid PV inverter. First, the output voltage of the PV(photovoltaic) array across a boost chopper circuit is increased, then the increased voltage as a DC bus voltage of the PV inverter. The stress voltage of each switch is a half of the DC bus voltage in the PV inverter. The paper details the SPWM control strategy, and combined to dual-loop control strategy, the strategy improved the external characteristics of the PV inverter output voltage. By MATLAB/Simulink simulation, with the dual-loop SPWM control strategy, PV inverter output voltage essentially unchanged during load fluctuations. The results very the advantages of the PV inverter and the effectiveness of double-loop SPWM control strategy.