Abstract: Summarizing the compositions of tritium in precipitation since the 1960's,and ba sed on the decay law of isotope,the tracks of tritium evolution in groundwater formed in different terms was described. Comparing the composition of tritium of precipitation in different terms with the evolution of tritium in groundwater formed from precipitation,the authors get conclusion that the rate of decay of tritium in groundwater is slower than that of renew of the tritium in precipitation,and explain the age and orig in of the groundwater in theodos basin,according to the composition of tritium in groundwater in the Ordos basin.