The non-uniform flow field around the vehicle flying at a high speed bends the incident ray and imposes an imaging deviation on the imaging plane. The relation between the deflection angle and the refractive index gradient is derived theoretically, and a new method is proposed to compute the deflection angle when the ray is passing through the discrete refractive index field with a small incidence angle. Then fluid-dynamics computations under different flying conditions have been completed. After that the mean density field is converted into the refractive index field. Ray tracing is performed to obtain the deflection angle and the gradients along the propagation path. The results show that the computed results by our method are well consistent with those by the Runge-Kutta and the Snell method when the incidence angle is less than 30°; and at the same altitude, the imaging deviation does not grow as the flying speed increases under the small incidence-angle condition, but remains stable; the deflection angle is determined by the positive as well as the negative refractive index gradient region, and the negative refractive index gradient region may reduce the imaging deviation.