Compared with Line-Commutated-Converter High Voltage Direct Current (LCC-HVDC), the primary and secondary systems of modular multilevel converter based HVDC (MMC-HVDC) are more complicated. And control and protection function of sub-module controller (SMC) have great influence on the operation of MMC-HVDC system. This paper investigates the design method of SMC and sub-module (SM) steady-state platform. First, the structure of the primary system and the electrical parameters of the 21-level MMC-HVDC are designed. Second, the architecture of the control system and SM fault protection strategy are proposed, and the control program is developed and debugged. Then SM steady-state test platform and the relevant control and protection are designed. Finally, the steady-state test and the system test of the physical MMC-HVDC simulation system are conducted. The test result shows that the sub-module is of good property and SMC can communicate with upper control layer efficiently, control and protect the SM efficiently.