By using the data of NCEP, the physical characteristics of tropical st orm ‘Haima' (0421) before and after its transformation are analyzed. The results show that before its transformation, ‘Haima' had a warm core and a sym me tric structure, and after transformation, the storm became more asymmetric and f rontal. In this process, the cold air invaded into storm from the southwestern p art at lowlevel layer. The relative vorticity changed from symmetric positive v alue at all levels beneath 200hPa to asymmetric structure. In the mean time, the mean relative vorticity and divergence in 24h in middle and low levels became b igger and layer became thicker. The height of vertical velocity center changed f rom 600hPa to 500hPa. There were two vaporchannels because of the eastward trough, one came with southwestern flow, and th e other came from the South China Sea with southern flow.