Based on the experimental testing and computational fluid dynamics analysis， a study on the effect of structure and parameters of an engine annular cooling fan on its aerodynamic performance was carried out. Firstly， the measurements of aerodynamic performances of an annular fan and an ordinary fan were conducted respectively， and the performance difference between the two type of fans was analyged. According to the test conditions of aerodynamic performance of the annular fan， a calculation model was developed and used to calculate the aerodynamic performance of the tested annular fan， which was verified by comparing the measured and the calculated data. By using the calculation model， the influence of circular ring structure and dimension parameters of annular fan on its aerodynamic performance was analyged. Results show that the fillet structure of annular fan’s circular ring can improve the static pressure efficiency. The relative axial dimension parameters of the circular ring have great influence on the aerodynamic performance of the annular fan， and each annular fan has a blade exposure rate to make the static pressure efficiency achieve the best. However， the relative radial dimension parameters of the circular ring have little influence on the static pressure efficiency of the annular fan.