Abstract: Aased on the two-dementional slab-symmetric model of cumulus cloud given by the authors, the develoFment of cumulus cloud and its precipitation in environments with and without stratiform cloud has simulated numerically in the nearly same atmospheric stratification. The results show that the existence of stratiform cloud has significant effect on the development of cumulus cloud and the increase of its precipitation. The rainfall may increase for several to tem times. It is belived that the convective-stratiform mixed cloud system may be important for producing heavy rainfall which fact has been observed in Mei-yu frontal cloud system in recent investigation.