Abstract: The reactor coolant pump casing of weld joint between steam generator outlet nozzle and reactor coolant pump is made by coarse-grain austenitic material. The weld joint is with thick wall, ultrasonic attenuation and grain scattering, the development of ultrasonic technique is very difficult. In this study, a special design is adopted to develop an automatic ultrasonic inspection system from inner surface of the steam generator outlet nozzle, and this system has been applied to the pre-service inspection of AP1000 Nuclear Power Plant in China. The results show that this system could fully meets the on-site inspection requirements, and the inspection results are in good agreement with the factory results.