The heavy rainfall caused by typhoon Morakot (0908) with 24-h accumulative precipitation larger than 1000 mm in Taiwan of China is closely related to the slow moving speed of Morakot. Using real-time NCEP-GFS global analysis data (0.5°×0.5°), synoptic charts of surface observations in Taiwan area from Central Weather Bureau (CWB) Typhoon Data Base and numerical simulations of the Weather Research and Forecasting Model (WRF), the mechanism for the slow westward movement of typhoon Morakot around Taiwan Island is investigated. Results show that the weakening and eastward retreat of the western Pacific subtropical high and the Fujiwara effect of typhoon Goni (0907) are the main environmental factors that affected Morakot's slow movement. The structure change of Morakot is also an important reason for its slow westward movement. It is found that the terrain over the Islands of Taiwan also induced several lows within the typhoon circulation system and increased its asymmetry. The terrain-induced low that developed on the western side of the Central Mountain Range replaced the original Morakot center on the eastern side, resulting in a westward discrete track. During this process, the Morakot structure experienced complex changes. Its positive vertical vorticity column changed from vertical to tilting position and then returned to vertical, and its lower layer circulation experienced splitting and re-organization processes. The above changes were largely responsible for the slow moving of Morakot. The impact of these mesoscale vortexes on the environmental steering flow is further investigated using the Barnes filter method. It is found that the mesoscale disturbance, although a small amount, is favorable for the increase in the westward steering flow. However, the percentage of the total environmental steering flow accounted for by the mesoscale disturbance increased from 7% to 26% when crossing Taiwan Island, indicating the important role of structure change related to the terrain in Morakot's slow westward movement.