A down hole main controller based on MCU is designed for RSS tools (rotary steerable system).The detailed functions of the main controller, the selection ofMCU, hardware design, and the software design based on RTOS (Realtime Operation System) are introduced. The module utilizes ADUC845 which has a C51 core, and an embedded RTOS,SMALL RTOS51, which is suitable for C51 MCU, is transplanted to implement all the functions needed with proper design of the thread tasks and semaphores. The realtime performance meets the requirement too. Experiments and field applications have proved that this main controller can guarantee steady and stable control of the RSS tool in harsh downhole environment,which proves it has brilliant application prospects.