Abstract: The development of gas drilling is restricted seriously because of frequent failure of drilling tools.To study the fatigue failure cycle of drill string,the whole drill string was taken in deep-vertical drilling as research subject.Based on Forman model in drill string fatigue life prediction,and taking consideration of the dynamic characteristics of drill string and the effect of local thermal factors on drill string fatigue life in gas drilling,the calculation equation of drill string fatigue life in gas drilling was established.Taking a real borehole in the west of Sichuan province as an example,the fatigue life of drill string in gas drilling was calculated with the established equation,then compared with that in mud drilling.Consequently,the relationship model of the fatigue life in gas and mud drilling was obtained.The results showed that the fatigue life of drilling tools was very sensitive to the initial crack.The initial crack develops larger,the fatigue life drops sharper.The fatigue life of drill string in each interval in gas drilling was obviously lower than that in mud drilling.