Abstract: In order to solve the problem of sensitivity loss due to the curvature of workpieces during the ultrasonic testing of circumferential butt welded joints in thermal power plant, the contact state between plane probe and curved workpiece is theoretically analyzed. The sensitivity contrast test was performed for the CSK-Ⅱ A test block and RB-C test block with different curvature and thickness. The results show that the probe's frequency, k value, wafer size, the type of coupling agent and the radius of curvature of the workpiece all affect the sensitivity of ultrasonic testing. The wafer size of the probe and the radius of curvature of the workpiece are the most important factors. The curvature workpieces reduce the effective area of contact with the probe, thus reducing the transmissivity of the defect echo sound pressure and leading to the loss of sensitivity of the workpiece inspection. In this paper, the correction coefficient of the surface coupling compensation of RB-C test block, which is commonly used in the ultrasonic testing of the circumferential butt welded joints of thermal power plant, is obtained through experimental research.