Modular multilevel converter (MMC) has been widely used in medium/high-voltage direct current transmission, motor drive, and renewable energy power generation. The reliability requirements of the MMC system, which is composed of numerous sub-modules (SMs) as building blocks, are getting crucial. However, the testing for reliability of SMs in a full MMC system is time-consuming and costly. In this paper, a mission profile emulator for SMs in the MMC system is therefore focused; by using this mission profile emulator, the SMs can be tested individually without building a full MMC system. Simulation results have shown that arm current and capacitor voltage of the SM under test agree well with the SM in a complete MMC system. A scaled-down experimental platform is built and experimental results have proved the validity of this emulator.