Abstract: The relationship between deuterium and oxygen 18 in atmospheric precipitation shows the certain law.This paper analyaes the impact factors for fractionation coefficient in Rayleigh fractionation,gives the differential equation models of stable isotope composition in precipitation in the open system and closed system,and discloses the variation law of deuterium and oxygen 18 in precipitation.And the relationship between deuterium and oxygen 18 in precipitation and clouds vapor is deducted.The result shows that it is not simply linear relation between the deuterium and oxygenl8 in precipitation.