Abstract: In this paper, the worn casing as used in a certain domestic ultra-deep well oil field is studied.And the full-scale casing abrasion test machine is used to simulate the drilling conditions and drilling fluid performances in order to analyze the wear statuses of the high strength casing in several different drilling fluids.The test results reveal the statuses as follows.When all the other conditions are the same, the casing worn-groove depth and the wear volume as separately caused by the oil-based, the hi-density organic salt, the hi-density saturated brine and the high-performance water-based drilling fluids are slightly different;the initial wear coefficient of the saturated brine drilling fluid is larger than those of the other three drilling fluids, then it drops sharply, and finally, the wear coefficient gets in consistency with the other three ones.Among the said four drilling fluids, the friction factor of the oil-based drilling fluid is small, and relatively stable.In case of high content of the barite powder, the said friction factor will be probably getting increased dramatically at the late wearing stage.