Abstract: The paper analyzed the physical mechanism that caused Typhoon 9806 to move toward north under the weak environmental condition field and to turn to the west under the strong environmental condition. It is concluded that the typhoon formed an asymmetrical structure under the weak environmental condition field and its physical elements such as the variation of height will add to the environmental condition field and change Typhoon 9806's tracks when it move into the strong environmental condition field. So probably the alteration of the weak-to-strong environmental condition field is the important reason for causing the track's mutation of typhoon 9806.