There are a large number of three-phase unbalanced loads in the distribution network and microgrid, which can cause three-phase unbalanced voltage problem. In order to solve this problem in microgrid, this paper proposes a novel three-phase voltage unbalanced compensation strategy for three-phase combined inverter based on Linear Active Disturbance Rejection Control (LADRC). Firstly, the paper analyzes the main reason leading to three-phase unbalanced voltage problem. And the three-phase voltage unbalance degree is defined. According to the Kirchhoff laws, a dynamic mathematical model including the known and unknown disturbance of microgrid and three-phase combined inverter is established to design Linear Extended State Observer (LESO). LESO is proposed to observe disturbance caused by three-phase unbalanced loads and the perturbations of internal parameters. Then the observed values are utilized as the control variables in feedforward compensation part. Finally, three-phase unbalanced voltage can be adjusted by using a feedback approach based on linear proportion-derivative (PD) controller. In order to verify the effectiveness of the proposed compensation strategy, a simulation model of the control system is established on the MATLAB/SIMULINK platform. Then, several different three-phase unbalanced loads conditions are designed. The proposed LADRC control method is compared with traditional PI double loop control method. The final experimental results verify that LADRC has a prior performance on reducing three-phase voltage unbalance degree, and getting lower total harmonic distortion (THD). Under different three-phase unbalanced loads conditions, the capacity of three-phase combined inverter carrying three-phase unbalanced loads is improved.