Due to lack of debugging method of flexible HVDC system, a kind of automatic debugging method used for flexible HVDC sub-module based on modular multilevel converter is put forward in this paper, and a test device including manager unit, MMC control and simulation unit, sampling unit and energy supplying unit is designed. The energy supplying unit can control energy output to provide power supply for sub-module. The MMC control and simulation unit can simulate valve control protocol to control sub-module to realize function test. The sampling unit acquires related voltage state of sub-module. The manager unit combines the sampled voltage with communication state for making test judgment, so as to complete automatic debugging of sub-module. Engineering practice shows that the method is fully applicable to the requirements of site acceptance test and factory acceptance test.