With the increasing voltage level of power systems, the number of serial sub-modules in the bridge arm of a modular multilevel converter (MMC) increases, and the hardware costs increase. This restricts its development in the direct current transmission system. In order to solve these problems, a new type of converter is proposed by analyzing the working principle of a multi-level converter and the existing gradationally controlled modular multilevel converter (GC-MMC). In order to solve the problem of capacitor voltage balance of the new inverter, a new voltage regulation algorithm suitable for the new inverter is proposed. Finally, in the Matlab/Simulink environment, a flexible HVDC transmission system based on the new inverter's double-terminal SCM unit value control is built and verified. The simulation results show that the output level of the new converter is more than that of the common MMC, and the quality of the waveform of the output AC and DC side can meet the requirements of DC transmission. Through the cost calculation of the new inverter and the common MMC respectively, the results show that the construction cost of the new inverter is much less than the common MMC. This work is supported by the National Natural Science Foundation of China (No. 61803219).