A statistical analysis of the causes, the spatial and temporal distribution characteristics for unusual tracks of tropical cyclones (TC) passed or close to Taiwan is performed, from 1949 to 2008, using tropical cyclone data and observational data, such as synoptic chart, satellite cloud picture and so on. The result reveals that the consociation of ambient fields and the Taiwan topographic forcing results in unusual tracks. In specific ambient fields, the Taiwan topographic forcing leads to changes in the structure of TC entering into this area , as well as the fields of pressure and wind around, and probably causes induced topographic trough or depression. The topographic forcing and the ambient fields play different roles in the unusual track, primary or secondary, when the TCs turn left, turn right, loop or are replaced by the induced depression, while the abnormality occurs in different regions.