Neutral point voltage balance of a three-level compensator produces insufficient compensation of zero-sequence current in a three-phase four-wire grid. This will affect the effectiveness of unbalance control. To solve this problem, this paper proposes a combined switching state control strategy after researching the relationship between zero-sequence current and neutral point voltage with regard to different kinds of voltage vectors. According to the ability to compensate zero-sequence current of all switching states, a combined scheme of 13 switching states composed of zero vector, short vector and middle vector is selected to practice model predictive control. The zero-sequence current compensation ability of different combination switching states under the neutral point voltage balance is compared by simulation, and the switching state proposed in this paper is adopted to control the unbalance. The results verify the effectiveness and accuracy of the combined switching state theory and control algorithm. This work is supported by National Natural Science Foundation of China (No. 51607159 and No. 51707176).