Abstract: In the process of building thermal power plant,the proportion of the small diameter pipes which need radiographic test is large. According to the distribution and characteristics of common defects in small diameter pipes of thermal power plant, a research was conducted by using radiographic test and phased array to detect natural defects and artificial simulation defects in the small diameter pipe in order to verify the possibility of using ultrasonic phased array test to replace the radiographic test in detecting small diameter pipes. The results show that it is possible to replace the radiographic detection with phased array detection by selecting reliable equipment and reasonable testing parameters.