Aiming at the model of the three-phase four-leg (3P4L) standalone inverter connected to the nonlinear load of the single-phase rectifier bridge, an unbalanced harmonic compensation strategy is proposed. It is based on Linear Active Disturbance Rejection Control (LADRC) and adds harmonic compensation information and N-phase control information to modify its control signal. The proposed control strategy presents faster transient speed and better disturbance rejection, so it can adapt to a wider range of load changes and worse working conditions. In addition, the tracking differentiator (TD) in N-phase control provides a differential signal with a high signal-To-noise ratio, which is also of great significance in engineering practice. Finally, the effectiveness and disturbance rejection ability of the proposed unbalanced harmonic compensation strategy are verified by simulation and experiment.