Abstract: Two semi-asymmetric flow patterns of typhoons are chosen to qualitatively determine the effect of exchange of horizontal momentum between inflow and outflow layers on the motion of typhoons. The results show that only the asymmetric flow component (residual after azimuthal mean flow has been removed) could cause a net momentum input into or output from a typhoon and therefore contribute to the changes in speed and direction of the typhoon movements. A typhoon with major inflow and/or outflow channels on its right (left) side would tend to accelerate and turn left (decelerate and turn right); On the other hand, a typhoon with major inflow and/or outflow channels in the front (rear) semi-circle would tend to accelerate and turn right (decelerate and turn left).