The end effector faces with the problem of sliding movement along workpiece surface due to the structure deformation of robot under the external forces. The property of robot drilling redundancy was exploited to improve the hole position accuracy by optimizing the robot posture to restrain the end effector sliding. The first and last optimal drilling postures could be chosen by finite element simulation. Firstly, the robot postures was calculated by inverse kinematics solution after the optimized range of redundancy freedom was calculated and dispersed. Then, the sliding deformations of the robot in different postures were calculated and compared using the robot simulation model, so that the optimal drilling posture, in which the robot had the minimum deformation, could be obtained. In order to determine other drilling postures,a posture smoothness algorithm based on the first and last optimal drilling postures was proposed, thus guaranteeing the hole position precision and increasing the robot joint motions efficiency. Some experiments were designed to validate the feasibility of the proposed method. Results show that the hole position precision is effectively controlled to be within 0.2 mm and the drilling efficiency increased from 5 holes/min to 6 holes/min. The required precision and high efficiency of robot automatic drilling are realized.