Abstract: In order to improve the detection accuracy of phased array ultrasonic testing system for large components such as wind turbine blades, four kinds of rectangular outlet nozzles with different structures were designed in this paper. The flow characteristics of each nozzle are compared and analyzed by numerical simulation. It is found that the nozzle with the inner surface of the flow channel designed by using the Witozinsky curve as the guide has the slowest beam divergence and the most stable jet. In order to reduce the processing cost, the ideal Witozinsky curve was simplified as a piecewise broken line based on the Douglas-Pucker algorithm, and the experimental verification was carried out. The results show that the performance of the simplified nozzle is close to that of the nozzle using the Witozinsky curve, and the jet stability is good, which has practical application value.