The equation of wave action of spiral Rossby wave in the baroclinic atmosphere has been derived with WKB method. If the background flow is time-independent, this equation is conservative.Based on this conservative equation of wave action the necessary condition of instability in spiral Rossby wave case has been obtained. This condition is that the generalized gradient of the potential vorticity is equal to zero at least in a spacial curve within the region considered in the baroclinic atmosphere. This unstable criterion is the same as Other authors, such as Kuo, Charney and Stern and Eady under some simplified circumstances. By means of the equation of wave energy, it is indicated that the evolution of spiral Rossby wave depends upon the distribution of the wind speed and the shape of the trough or ridge line. The contributions of β and the jet are also discussed. The equation of wave action of spiral Rossby wave in the baroclinic atmosphere has been derived with WKB method. If the background flow is time-independent, this equation is conservative. Based on the conservative equation of wave action the necessary condition of instability in spiral Rossby This condition is that generalized gradient of the potential vorticity is equal to zero at least in a spacial curve within the region considered in the baroclinic atmosphere. This unstable criterion is the same as other authors, such as Kuo, Charney and Stern and Eady under some simplified circumstances. By means of the equation of wave energy, it is indicated that the evolution of spiral Rossby wave depends upon the distribution of the wind speed and the shape of the trough or ridge line. The contributions of β and the jet are also discussed.