In this paper, the instability of shearline perturbations in the JiangHuai mei-yu period is studied and the principal results are obtained as. follows: 1) the growth rate of zonal perturbations is far smaller than that of symmetric perturbations; 2)the growth rate is strongly dependent on wavelengths and stratification stability; 3)the condition for the maximum growth rate of symmetric perturbations is R_i<-0.1, and that for the minimum growth rate is R_i<0.92 or 1/4; and 4) the wavelengths of the most unstable waves which initiate a severe convective storm or a thunderstorm are about 200—400 km.