Abstract: The sound velocity in T91/P91 steel was calculated and tested, then by the reflection law of sound and geometric relationships, the change of K values, depth and horizontal distance was calculated when the T91/P91 steel was inspected by common probes with different K values. The weld joint of T91/P91 steel pipe and the weld joint between T91/P91 and low-alloy steel could be inspected by the common ultrasonic probe and test block.