Based on the Tropical Cyclone (TC) data and Japanese 25-year reanalysis data of Japan Meteorological Agency from 1979 to 2008, compositing and comparative analysis was firstly performed to study the intensifying and weakening TC during the Extratropical Transition (ET) over China.The analyses reveal a nice corresponding relationship between the variation of TC's intensity after ET and initial strength of the westerly upper-level trough. Then TC Haima which made landfall on China in 2004 was taken as an example, the evolution of its transition process was reproduced by the mesoscale atmospheric model. The authors modified the intensity of upper-level trough in the initial fields using the method of piecewise potential vorticity (PV) inversion and investigated the impacts of different troughs on Haima's re-intensification process during ET. The results indicate that Haima moved quickly (slowly) when the trough enhanced (weakened), furthermore, the deeper (shallower) the upper trough is, the stronger (weaker) the Haima re-intensifies; the deeper trough shows wide and strong upper-level jet and enhanced divergence, which is favorable for Haima's redevelopment; besides, the deeper upper trough was accompanied by stronger positive belt of PV, the upper-level positive PV propagated downward to lower levels to result in the growth of Haima's positive PV and the intense development of low-level baroclinic frontal zone, finally, the low-level cyclone was reinforced obviously.