It is important to maintain the neutral-point(NP) voltage balanced for the three-phase four-wire three-level neutral-point clamped(NPC) inverter. In this paper, after detailed discussion, a mathematical model of the neutral-point voltage are derived. Then a novel control strategy is proposed based on the disassembly of zero level(O Level) to maintain the neutral-point potential. A variable named neutral-point control margin(NPCM) is defined to represent the disassembly margin of each phase. The zero level of one of the three phase is disassembled quantitatively after calculation. Furthermore, the neutral-point voltage can keep balanced while the average output voltage remains unchanged. The proposed control strategy is verified by simulation and experiments.