It is proposed in this paper that there are two cireular regions of a typhoon where unstable spiral waves can occur. Furthermore, the instability mechanism is different in each region. The locations of these regions coincide with the inner and outer spiral rain bands respectively. Analysis indicates that in the inner unstable region, some eigenwaves are reflected by the strong nonuniformity of the typhoon’s basic flow and a“quantum condition” is established correspondingly, which gives the eigenfrequency and growth rate. The estimated growth rate, spiral arm number, wave length, polarization and thermal structure for the proferentially growing eigenwaves are comparable with the observations of inner spiral Cb cloud bands. It is proposed in this paper that there are two cireular regions of a typhoon where unstable spiral waves can occur. Moreover, the instability mechanism is different in each region. indicates that in the inner unstable region, some eigenwaves are reflected by the strong nonuniformity of the typhoon’s basic flow and a “quantum condition” is established correspondingly, which gives the eigenfrequency and growth rate. The estimated growth rate, spiral arm number, wave length, polarization and thermal structure for the proferentially growing eigenwaves are comparable with the observations of inner spiral Cb cloud bands.