The conventional inverter frequency conversion operation control method lacks quantitative analysis of the power difference between various modules of the converter, resulting in a high switching frequency for inverter frequency operation control. Therefore, a PLC-based study on modular multilevel inverter frequency conversion operation control is proposed. According to the principle of voltage modulation, the triangular carrier wave is divided into two modules, which are used to adjust the voltage of the converter. Combined with the adjusted voltage of the low - frequency operating submodule, the frequency conversion operation error of the multilevel converter is calculated. Based on the quantitative analysis of the power difference between various modules of the converter and PLC technology, a modular multilevel converter variable frequency operation control model is constructed to control the variable frequency operation of the converter. The experimental results show that the switching frequency is as low as 251 Hz under the variable frequency operation control method of modular multilevel converter, and this method can optimize the fluctuation of switching frequency.