Z-source inverter main-line voltage has a fluctuation and its frequency is same with the second harmonic of output sine wave, it will increase the second harmonic content of output which affecting the performance of the inverter. Introduced the topology and control method of Z-source inverter. By modeling the Z-source inverter, analyzed the reason of DC voltage fluctuation. Proposed the method to weak the fluctuation based on circuit design. By Comparing the simulation results of main-line voltage under various circuit parameters, verified the effectiveness of this method.