In order to improve the accuracy of outfield sound source identification in automobile wind tunnel, the deviation caused by the difference between actual flow field and empty wind tunnel is analyzed. Focusing on the velocity distribution of flow around vehicle and jet shear layer, numerical simulation and experimental verification were carried out on 3 kinds of flow fields, including free space where vehicle is located, empty wind tunnel and wind tunnel with actual vehicle. The amounts of sound drift in wind tunnel with or without vehicles were predicted by using infinite thin shear layer model and layered flow field sound propagation model. The predicted sound drift quantities were compared with actual full-scale acoustic wind tunnel sound propagation test results. The results show that，after vehicle was placed in the wind tunnel, the flow field velocity distribution in core area of potential flow was uneven due to obstruction of the vehicle, and then the flow field in shear layer was disturbed by flow field around vehicle. After stratified correction of the actual flow field, the accuracy of sound drift quantity at rear sound source of vehicle body is greatly improved, which can effectively improve the identification accuracy of the actual wind tunnel.