Diagnostic analysis was performed on Typhoon Nari (2001), which exhibited abnormal structural and intensity changes when crossing Taiwan Island, on the basis of the 2001 Tropical Cyclone Yearbook compiled by the Chinese Meteorological Administration, the United States National Center for Environmental Prediction (NCEP) final (FNL) 1°×1° global grid data, and temperature of black body (TBB) data from the Japan Meteorological Agency. The following results are reported: (1) Nari’s cloud structure changed from circular to semicircular with convective clouds developing in the western area and dissipating in the eastern area of typhoon circulation during its landfall process and maintained such an asymmetric structure over the island. (2) When crossing the island, a lower-layer jet (LLJ) stream in Nari’s western circulation strengthened and extended counterclockwise from north to south. The increase in angular momentum of the tangential wind was favorable for strengthening vortex circulation. In addition, the water vapor flux convergence near the LLJ and the strong positive vorticity on the left of the LLJ axis helped to maintain the cyclonic vortex. (3) Taiwan topography, including the Taiwan Strait, had a pronounced effect on the structure and intensity of Nari. The windward slope-forced lifting intensified ascending air flow in Nari’s western circulation; however, the sinking on the leeward side restrained the convection in its eastern circulation. These factors were primarily responsible for the formation of Nari’s semicircular asymmetric structure. Moreover, the channel effect of Taiwan Strait benefited the strengthening and stretching of the LLJ stream in Nari’s western periphery. (4) A diagnosis of kinetic energy budgets revealed that the kinetic energy of Nari was generated from the solenoid effect. In addition, horizontal convergence movement provided kinetic energy for Typhoon Nari during its landfall process.