Abstract: The spiral cloud bands usually are regarded as one type of wave named the vortex Rossby waves among which the double arm corresponds to the tangential wavenumber-2, and the single arm corresponds to tangential wavenumber -1. At the present the vortex Rossby wave’s mechanism still needs further studies. This article, by applying the barotropic fluid vorticity equation of cylindrical coordinates, studied the bartropic instability and change process of the vortex Rossby wave in a typhoon, and the results show that when the vorticity gradient of the basic flow is below zero, the vortex Rossby wave of wavenumber-1 may absorb energy from the base flow and then grow very quickly, which is attributed to the strong advection effect that vorticity is transferred from basic flow to fluctuation. Because the base flow entangles tight the perturbation, the radial velocity fast turns to be very small and therefore the unstable growth rate will be limited. In addition, the speed of fluctuation’s development is related to its radius. The fluctuation near the vortex center develops firstly and then disseminates while the non-centric fluctuation develops later. Also， the research indicates that the spiral cloud bands of a large scale typhoon develops faster than those of a small scale typhoon do.