A modular multilevel converter (MMC) parallel system based on half-bridge sub-module (HBSM) is applied in order to build the integrated test equipment for large-capacity offshore wind turbine generator.The operation principle of the device and its primary parameter selection method are elaborated in detail. Power electronics technology is adopted to send low/high voltage signals. The voltage signal is synthesized from the reference voltage, the fundamental frequency voltage difference, and the harmonic command voltage, which are given by split-phase V/F control, and consequently the amplitude, phase and frequency of the output AC voltage can be adjusted. PSCAD /EMTDC simulation results show that the system designed can meet the grid-interconnection requirements for both fault ride-through and grid adaptability tests, including high/low voltage ride-through, voltage imbalance, frequency deviation, harmonic output, voltage fluctuation tests for large-capacity offshore wind turbines.