There will be inevitable wear between the drill pipe and casing because of the dog leg, while hardness of casing is one of the principal factors influencing the wear resistance. Wear experiments of N80, P110, VM140, and TP140 grades casing were done in 14g/cm3 polysulfide water based drilling fluid to measure the wear parameters such as wear efficiency, friction coefficient，and casing wear influence factor(CWIF) was proposed based on the measured wear parameters. It is found that the higher the hardness, the smaller the CWIF，and the better wear resistance performance of casing. N80 grades casing has the worst wear resistance, TP140 grades casing has the best wear resistance, and the second is P110.The CWIF of N80 is 208 percent of TP140. The proposed CWIF is the product of wear efficiency and friction coefficient，which is more reasonable to be used in evaluating the wear resistance compared to wear efficiency.