In order to analyze and improve the voltage asymmetry problem clearly and intuitively, this paper adopts a harmonic linearization method from the order impedance angle to establish a three-phase off-grid inverter with unbalanced load and phase separation voltage control mode under unbalanced load. The positive and negative sequence impedance models are compared and analyzed under different PI control parameters and different operating conditions. The analysis shows that the PI control parameters have a great influence on the positive and negative sequence impedance characteristics, and the impact of the operating conditions is small. Then, under three different operating conditions, the single-sequence dq control mode and the non-positive-negative-sequence separation voltage control mode are simulated and compared. The obtained sequence impedance characteristic curve and output waveform show that when the single-sequence dq voltage control mode is adopted, the output voltage amplitude of the off-grid inverter is unbalanced. When the positive and negative sequence separation voltage control mode is adopted, since the negative-sequence equivalent output impedance of the three-phase off-grid inverter is small at the fundamental frequency, the voltage drop caused by the negative sequence current flowing through the balanced load is also small. The output voltage amplitude of the inverter is basically balanced. Finally, the simulation verifies the effectiveness of the sequence impedance model without positive and negative sequence separation voltage control mode. This work is supported by the National Key Research and Development Program of China (No. 2017YFB0902000), National Natural Science Foundation of China (No. 51677063), and Postdoctoral Innovation Talents Support Project (No. BX20190109).