A method for tilt measurement and compensation of a GNSS RTK surveying instrument based on a GNSS receiver and an IMU sensor is proposed. Based on the algorithm principle of the Kalman filter, the prediction and the correction of the designed multi-dimensional unknown state variable are cycled for multiple times to make the multi-dimensional unknown state variable approach the true value as much as possible. Then, based on the multi-dimensional unknown state variable, the latest three-dimensional attitude angle from the XYZ coordinate system to the NED coordinate system and the latest rotation matrix can be obtained efficiently and accurately. Finally, the coordinate of the antenna phase center of the GNSS receiver is reduced to the coordinate of the bottom end of the pole by a geometric transformation. In addition, no factory calibration is needed as the self-calibration can always be done during the operation process.