Abstract: Compared with experiment investigation, UT simulation technique can improve the comprehension of basic principle of the ultrasonic wave generation and imaging. It is more economy. When the mathematic module established, it is convenience to do the investigation by change the different parameters used to design or optimize ultrasonic inspection technique. This paper describe the application of UT simulation technique in the ultrasonic testing of reactor pressure vessel (as an example, flange to shell weld), in order to illustrate the process of simulation on ultrasonic inspection and the results of simulation.