By using the regional spectrum model reanalysis data from the Japan Meteorological Agency with a horizontal resolution of 20 km and a time interval of 6 h, in addition to nondivergent and irrotational wind components derived through harmonic-cosine spectrum expansion, wind structure features were compared prior to and following the rainstorm amplification of typhoon Bilis (0604). After Bilis landed on the Chinese mainland, the rain amplification was noticeable on its southeast quadrant, particularly from 18 UTC July 14 to 12 UTC July 15, 2006. The analysis results show that the variation amplification of the entire wind speed and the nondivergent wind component were less than that of the irrotational wind component. In detail, before the rainstorm amplification occurred, the irrotational wind component of the southeast quadrant increased on the lower and upper levels, and the distance between convergence center and the center of Bilis narrowed. Thus, the vertical upward motion strengthened. During the period of rain amount amplification, the irrotational wind of the lower and upper levels increased as well. Therefore, the variation of irrotational wind is relevant to the variation in rainfall intensity. The variation of irrotational wind and its resultant divergence were the main factors for the rainfall amplification. The enhancement/diminishing of irrotational wind and divergence were always accompanied by an increase/decrease in rainfall amount. The tendency of nondivergent wind was opposite that of the rainfall amount, and although the whole wind speed was determined mainly by the irrotational wind during the rainfall amplification, it was affected more by the nondivergent wind during the rainfall weakening process. The analysis for the irrotational and nondivergent winds can indicate additional wind variation characteristics. The conventional analysis method for the entire wind cannot reveal the changes of irrotational and nondivergent winds and their relative importance when rainfall occurred. From the wind decomposition results, we conclude that before and after the rainstorm amplification of Bilis, the change in wind field was reflected mainly on the irrotational component, which was sensitive to the change in rainfall. Thus, this method is effective for predicting wind and rainfall distribution for the landed typhoons.