Abstract: In this paper, the relationship betweeh Tess's track and the surrounding field, stream function and potential function is discussed, and also the change of typhoon Tess's track is analysed and calculated using the diagnosis equation for the motion of typhoon, which is deduced by authors in another paper. Particularly, the reasons for typhoon Tess's track to right-bent at OOOOZ 5 September are given a full account The result shows that it is helpful to use the diagnosis equation for the motion of typhoon during analysis and forecast.