Abstract: Using the T63L16 analysis data with the resolution of 1.875×1.875 degree of latitude and longitude obtained from National Meteorological Center (NMC) and the real central position information of tropical cyclone numbered by NMC,the environmental geostrophic basic flow at 126 times of 25 tropical cyclones in 1996 are calculated. The vertical distribution features of the environmental geostrophic basic flows are analyzed. Besides,the deviation of real TC tracks from the environmental geostrphic basic flow (referred as steering flow hereafter,namely the deviation between the real central position of TC and the position calculated according to the steering flow) is also investigated. The result shows that the steering deviation would be different if the domain used to calculate the steering flow is different. The present paper obtains the optimum domain size to calculate the steering flow. It is found that the steering deviation has relationship with the velocity of steering flow and the initial latitude and intensity of tropical cyclone itself,and that tropical cyclone motion has relationship with the vertical shear structure of environmental geostrophic flow. The result also shows that the optimum steering flow is the deep layer averaged basic flow from 1000 hPa to 200hPa. Having the knowledge of these principle and features would help to make accurate forecast for tropical cyclone motion.