Abstract: With the development of aircraft, higher requirements are put forward for the accurate prediction of aerodynamic and operating characteristics under different wind tunnel simulation conditions. The flow quality of wind tunnel is one of the main factors affecting the accuracy of wind tunnel test data. A Mach 5.5 nozzle used as the research object in this paper, the non-uniformity of nozzle outlet flow field is studied. The structure of weak waves in nozzle is captured by numerical simulation. The influence of different factors on the quality of nozzle outlet flow field is analyzed, including the profile design method, profile machining accuracy and step joint at profile splicing. Combined with the comparison and analysis of the wind tunnel calibration results, it is concluded that the main factor affecting the quality of nozzle outlet flow field is the step joint at profile splicing. An improvement strategy of segment the nozzle properly is proposed to improve the quality of flow field.