Abstract: A ccording to the cloud physics model,the paperanalyses the variations of stable isotopes in raindrops under different conditions,inquires into the im pacts ofenrichment produced due to the evaporation of raindrops below cloud base and the exchange between stable isotopes of the raindrops and surrounding vaporon stable isotopic ratios in the raindrops.Foradrop: (1) Influenced by evaporation process,stable is otopic ratios in the raindrop increase with the falling distance. The less the humidity in atmosphere,the more obvious the range of increasing;on the other hand,if there is a difference bet ween stable isotopic ratios at the boundary layer of the raindrop and in surrounding vapor,the move of stable isotopic composition from high to low value will be produced.Two factors restrictthe variations of stable isotopic ratios in the drop in common.Their direction may be same,or different.Under the condition of saturation,the impact ofevaporation disappears,and the degrees of stable isotopic ratios in the drop are dependenton the exchange with stable isotopic composition in surrounding vapor.If the two processes all keep the equilibrium,namelys=100%,and Rv=Rl/α,then dRl=0,which shows that stable isotopic ratios don't change with the falling distance. (2) Generally,the atmospheric humidityis in direct proportion to dδD/dδ18O and Ex d.In any case,all dδD/dδ18O have a gradual value of equaling 8,and Ex d gradually tends to a constantunder the conditions of the high temperature and the large radii, which might be related to the appearance of "pseudo-equilibrium state". (3) In saturated at mosphere,the time of reaching isotopic equilibriumis short for small drop orin an environment with the high temperature;on the contrary,the time is long.