Abstract: In this paper, the dynamic model incloudes the effect of cumulus heating in the condition of given shear basic flow The result shows that the critical wave-length of baroclinic unstable disturbance shortens apparently When the cumulus heating amounts to 3.5°C/day, the scale of the developed disturbance coinsides with that of the mid-tropospheric cyclone. In the latter part of this paper, the frictional effect of cumulus convection is studied, and the results show the instability of disturbance too.But the cumulus convection, generally speaking, has damping effect on disturbance.