The data of numerical simulation are used to analyze the microphysical structures and the precipitation forming mechanisms for the spiral rain bands of typhoon. Two precipitation centers of different times are chosen as the cases being studied in the paper. The main microphysical characters of the spiral rain bands are as follows. The heterogeneous nucleation is very intense at altitudes 9-13 km, the ice crystal conversion ratio is several times higher than that in typhoon eyewall. However, the deposition processes of ice crystal and snow are very weak. The spiral rain bands are mainly produced by melting graupels (pgmlt). The graupel grows mainly through accretion with snow microphysical processes (dgacs).The peak value of the processes “dgacs” stays at altitude 8 km with big vertical vector. Compared with the stratiform clouds in the northern China, the deposition and accretion processes are very fierce in the typhoon spiral storms due to high moisture content. The profiles of the cloud water condensation processes in the spiral rain bands show a double peak type. The biggest peak stays at altitude 8 km in the cold cloud, and the secondary peak stays at altitudes 0.5-1.5 km in warm clouds.