Abstract: There are quite a number of typhoon landfalls in China in summer, and some of them go northward, weaken slowly, and then redevelop after their dynamic and thermal structures transform which is called extratropical transition, as a result of the interactions with the midlatitude systems. Meanwhile, in summer the subtropical high is active and frequently extends westwards. The physical mechanism of the interaction between the subtropical high system and the landfalling typhoon is still not very clear, which has become an obstacle to the forecast of landfalling typhoons. Typhoon Winnie (1997) was selected as an example, and we used the MM5 mode to reproduce the weakeningtransitionredeveloping process after its landfall. The piecewise PV inversion proposed by Davis et al. was used to alter the initial intensity of the subtropical high east of Winnie. Numerical experiments with different initial intensities of the subtropical high were performed to investigate the responses of landfalling typhoon Winnie. The results show that the strengthened subtropical high accelerated the northward motion of Winnie, and orrespondingly, cold air from the middleupper troposphere intruded into Winnie earlier than that in the control experiment, thus quickening the typhoon’s extratropical transition. Along with the forced rising of warm masses, intense frontogenesied appeared in the midupper troposphere, and the transformed cyclone redeveloped again. The changes in the initial intensity of the subtropical high directly altered the upper tropospheric circulation pattern over the center of the typhoon, which was found closely correlated with the intensity of the tropical cyclone.