The well drilling often leads to the wear of casing in the cased well.The reduced strength of worn casing threatens the safety of well testing and completion.By mean of the ring block drill pipe and casing wear tester,the wear resistance of casing in the water based mud was investigated,the wear efficiency and the friction coefficient of casing at different wear time was measured under the action of different contact force and rotating speed.The wear mechanism of casing was analyzed by SEM pictures of the worn surface.The results show that both the rotating speed and the contact force have bigger influence on the wear of casing.With the increase of contact force,the wear efficiency is increased,the friction coefficient is first increased and then decreased.With the increase of rotating speed,the wear efficiency and the friction coefficient of casing are all increased.The wear mechanism is composed of abrasive and fatigue wear on the casing surface under low contact force,while there are the adhesive,abrasive and fatigue wear under high contact force,but the adhesive wear leads to the most of casing wear.The decrease of the contact force between casing and drill pipe can reduce the wear of the internal surface of casing in well drilling.