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The Typhoon Fitow experienced four rainfall stages: typhoon peripheral precipitation, rainfall caused by the typhoon inverted trough and typhoon itself, rear spiral rain band rainfall, and rainfall originating from the interaction between outside circulation of the residual low pressure and the cold air. Using the data of surface observation, weather radar, and NCEP analysis, the rainstorm circulation forms of Fitow are analyzed. The east and west sides of high pressure, which guided the eastwards movement of typhoon, had different thermal properties. The east side was warm and deep, while the west side was cold and shallow. The shallow cold high pressure prevented the westward movement after landfalling, leading to the persistent rainfall under the typhoon inverted trough and in the rear spiral rain band. The approaching of typhoon Danas was beneficial to the enhancement of water vapor transfer with southeast winds. The interaction between typhoon and the osmotic cold air, which was different from the traditional detaining cold air with large vertical wind shear and a strong upper level jet stream, was discussed. The osmotic cold air invaded from the lower layer and affected the outside circulation of residual low pressure of typhoon. The strong northeast winds formed in the north of outside circulation of low pressure and converged with the east winds from the sea, which formed the coastal front. The spatial asymmetry characteristics of the osmotic cold air were obvious: convective available potential energy was high in the east and low in the west, and the vertical wind shear was high in the northwest and low in the northeast. The cold air had only small influence on the middle layer of the residual low pressure, so the southeast winds in the coastal area existed for a longer time. The interaction between high potential vorticity area in the middle layer and the coastal front on the surface provided good conditions for the residual low pressure rainstorm of extra tropical transition.