Abstract: In order to achieve more accurate comprehensive stress assessment of the flexible DC converter valve, aiming at the valve current with the second harmonic circulation in the valve, an improved operation test method for flexible DC converter valves is proposed. The method can meet the testing requirement of flexible DC converter valves in operation conditions with different characteristic currents at different power levels. Firstly, the stress analysis of the typical operation conditions of the flexible DC converter valve is carried out, focusing on the current characteristics of the valve. Secondly, combined with the stress analysis,considering that the bridge arm current contains a large second harmonic component when the converter valve is in operation without circulation suppression in the actual project, and based on the valve section test device of the flexible DC converter valve and no additional test cost, a test method for assessing valve current stress and voltage stress with the second harmonic circulation components is proposed via the modulation control strategy, and its operation mechanism is described. Finally, an actual test system is built and the method is verified. The test results show that the improved operation test method for flexible DC converter valves is feasible and effective.