Typhoon Sinlaku (2008) was a tropical system that affected many countries in East Asia. Besides the loss of life and economic damage, many scientific questions are associated with this system that need to be addressed. A series of numerical simulations were conducted in this study using V3.2 of the advanced research version of the Weather Research and Forecasting (WRF-ARW) model to examine the impacts of different terrain conditions and vortex structures on the eyewall evolution when Sinlaku was crossing Taiwan. The sensitivity experiments using different vortex structures show that a storm of the same intensity with a larger eyewall radius tends to induce stronger wind and rainfall at the outer part of the storm during the terrain-crossing period. This result suggests that the vortex contained with larger angular momentum is more favorable to reform a new eyewall from the contraction of the outer rainband after being affected by terrain. Based on these sensitivity experiments it is suggested that the topography and the tropical cyclone (TC) structure play important roles in regulating the outer tangential wind speed and modulating the unique eyewall evolutions for TCs passing Taiwan. A stronger vortex structure could lead to more precipitation at the outer part of the storm during the terrain influenced period, implying that the forecasters should pay attention to the storm intensity and also the storm structure which is an important dynamic feature that modulates the eyewall evolution and rainfall distribution of a landfalling storm.