In order to check the stress and operation reliability of modular multilevel converter valve, a new operation test topology and control method of modular multilevel converter valve is proposed under the condition of gradually increasing sub module voltage of high voltage and large capacity flexible DC converter valve. Firstly, the stress analysis of typical operation condition of the flexible DC converter valve is carried out. Secondly, combined with stress analysis, a new operation test topology of modular multilevel converter valve is proposed, and the mathematical model of the test topology is established, and its operation mechanism is expounded. At the same time, combined with the symmetry of the control parameters, the DC component in the control variables is eliminated in real time. Finally, the simulation model is built and verified. The results show that the new operation test topology and control method of modular multilevel converter valve are correct and effective. The proposed test topology reduces the voltage demand of DC test power supply, and its control method simplifies the control of test circuit.