Considering the characteristics of Fengyun-3(FY-3) satellite lifespan on orbit increased from 3 years to 5 years, the risk of extending lifespan was analyzed, and the reliability of scan driver system and receiver system which were critical items of microwave radiation imager was evaluated, taking corresponding measures from design and improving crafts reliability. Microwave radiation imager operates normally on orbit and provides effective reliability assurance according to these methods. Finally, the ideal of accelerated lifespan test was put forward at last.