The galvanized layer on both ends of the corrosion resistant alloys (CRA) casing surface is easily to be damaged by the conventional power tongs during working for the material of CRA casing usually used in the oil field is relatively soft,which will accelerate the hydrogen sulfide corrosion of the casing.In order to reducing the damage of hydraulic tongs on casing,and slow down the corrosion of hydrogen sulfide on casing,based on the working principle of the conventional hydraulic power tongs,the mechanics model of jaws was set up,and the specific factors affecting tongs tightening were determined.According to the tribological theory of fatigue,causes and process of wear of the casing by jaws were analyzed.Through the experiments of screwonoff by the jaws on oil pipe,the result shows that wear has obvious relationship with contact area,number of cycles,the mutual contact materials and other factors.Through the stress and strain analysis of contact area，it is shown that friction coefficient and contact load are the major factors influencing the casing wear by jaws,and the friction coefficient has more obvious influence on tensile strain.