The principle of a diode neutral point clamped three-phase three-level inverter with neutral line was analyzed, and the mathematical model of the main circuit is established on the basis of the operation principle. The oversample and multi-time calculation control strategy was proposed to reduce the typical delay in digital control and improve the system calculation accuracy, lower the inverter output voltage THD. In order to further improve the inverter output voltage waveform, the logic external execution was also proposed. The proposed control method was verified on a 20kVA inverter controlled by microchip processor IC30F5015 DSP. The experiment results indicated that the THD of the inverter output voltage is decreased considerably.