Abstract: The small-size branch joint (BOSS) weld is the weak point of the primary pressure boundary in nuclear power plants, and its monitoring is critical but difficulty in the daily and in-service inspection. This paper designs and verifies the ultrasonic phased array inspection technology for BOSS welds, by using the simulation technology, technical test and application verification, and also solves the difficulties of monitoring BOSS welds in daily and in-service overhaul of nuclear power plants. The method of design and verification can be used in similar ultrasonic phased array inspection process.