The cascade multilevel inverter generates positive, negative or zero voltage by controlling the switching state of each H-bridge unit. However, at the low speed stage, the imbalance between the electromagnetic torque and the mechanical torque of the motor will make the motor torque fluctuate greatly, and the sensitivity to the voltage amplitude and phase is higher. However, the traditional speed frequency controller mostly uses a phase shift transformer to output a fixed amplitude and phase voltage. This fixed output can not meet the needs of fine control of speed at low speed, resulting in a large fluctuation of speed frequency. Therefore, the low speed frequency control system of cascade multilevel inverter is designed and developed. The speed frequency sampling element and multilevel inverter working signal processor are modified, multiple phase shift transformers are added, the internal structure of the speed frequency controller is adjusted, and the drive circuit and protection circuit are added to the traditional system circuit to realize the connection of multiple hardware components, and the optimization of the multi-gear speed frequency control hardware system is completed. The speed frequency control target is determined according to the parameter standard of the cascade multilevel inverter gear. Using MT velocity measurement method, the initial value of speed frequency in low speed stage of cascade multilevel converter is accurately detected. The speed frequency control is calculated by comparing the control target with the initial control value and combining with the multi-phase shift transformer. Based on the calculation results of the control quantity and the relationship between voltage, frequency and speed, the frequency control command is generated and executed by the step wave control mode to realize the multi-gear speed frequency control function of the inverter at low speed stage. The results show that compared with the traditional control system, the frequency control deviation of the optimized design system is significantly reduced, and the speed regulation accuracy and speed regulation range are significantly improved, which proves that the designed system has better control function.