Around 30 October 2012, Hurricane Sandy made landfall along the New Jersey shoreline after its completion of extratropical transition and transformation into an extratropical cyclone. The strong gale induced a catastrophic storm surge, and caused 72 death and damage of more than $50 billion. In this paper, the evolutionary process and spatial structure of the Hurricane Sandy during its extratropical transition were investigated by using Weather Research and Forecasting(WRF) version 3.3.1 modeling results and National Center for Environmental Prediction(NCEP) Coupled Forecast System model version 2 reanalysis datasets(CFSv2). It is found that during the upper-level trough interaction on 29 October, Sandy gradually fused with a pre-existing mid-latitude low-pressure system, and finished the re-intensification. WRF modeling results showed that the second peak occurred mainly due to the enhanced vertical motion, reduced vertical wind shear as well as the supplement of potential vorticity resulting from trough interaction over the southeast of Great Lakes. The cold continental air from the back of trough was encircled within the warm core system cyclonically, forming the characteristic of warm seclusion. Around 30 October 2012, Hurricane Sandy made landfall along the New Jersey shoreline after its completion of extratropical transition and transformation into an extratropical cyclone. The strong gale induced a catastrophic storm surge, and caused 72 death and damage of more than $ 50 billion. In this paper, the evolutionary process and spatial structure of the Hurricane Sandy during its extratropical transition were investigated by using Weather Research and Forecasting (WRF) version 3.3.1 modeling results and National Center for Environmental Prediction (NCEP) Coupled Forecast System model version 2 reanalysis datasets (CFSv2). It is found that during the upper-level trough interaction on 29 October, Sandy gradually fused with a pre-existing mid-latitude low-pressure system, and finished the re-intensification. WRF modeling results showed that the second peak occurred mainly due to the enhanced vertical motion, reduced vertical wind shear as well as the supplement of potential vorticity r esulting from trough interaction over the southeast of Great Lakes. The cold continental air from the back of trough was encircled within the warm core system cyclonically, forming the characteristic of warm seclusion.