The cloud and the rain drop size distributions in stratiform clouds which are in the region of Mount Tai and Xinan River are calculated . The cloud droplet and drop (d≥80μm) and the raindrop size distributions are fitted by using N=AN₀exp(-ad+bd²-cd³). The observational and calculated results are well matched. The correlation coefficients are bigger than 0.93 and the significance level is 0.001. The calculated mean diameter, mean cube-root diameter, liquid water content and total concentration are all well matched with observational results. It has been proved that the water drop size distribution is representative.