Purpose: to achieve on-orbit servicing or off-orbit removal of non-cooperative targets in space, such as inactive satellites, space debris, especially slowly rotating targets, accurate measurement of relative attitude between the tracking aircraft and target aircraft is necessary. Method: firstly, used inverse depth parameterization to represent the coordinate value, angular altitude, azimuth angle and depth information of the camera, which could effectively complete the monocular vision attitude estimation under small parallax error; secondly, established a motion model and measurement model of the camera relative to the non-cooperative target; finally, achieved the relative motion attitude estimation between the camera and target based on single candidate RANSAC and EKF. Results: for three-axes stable targets, the attitude measurement accuracy during the approaching process is approximately 0.5°; for slowly rotating targets (constant velocity), the relative angle error is about 3.5%, with an average angular velocity error of approximately 0.1°/s. All these parameters can meet the requirements for relative attitude measurements of engineering spatial non-cooperative targets.