Aiming at the measurement problems caused by different design coordinate system and measuring coordinate system reference system, a method of measuring free surface based on differential confocal normal measurement registration was proposed.The method utilizes the characteristic that the zero points of the differential confocal curves correspond accurately to the measured points to achieve precise focusing of the measured points.The inclination angle of the measured points is measured precisely by the PSD (Position Sensitive Detector) at the focus position.Based on the inclination angle of the measured points, the ICP (Iterative Closest Point) Point to Plane method is used for point cloud alignment to improve point cloud alignment accuracy and significantly improve surface measurement accuracy.According to the theoretical analysis and preliminary experiments, the accuracy of PV measured by this method can reach 65nm, and the RMS can reach 10nm.The difference between the PV and the zygo interferometer measurement results is 7nm, the difference between the RMS and the zygo interferometer measurement results is 1nm, which means the measurement results have small errors.The method provides a new technical approach for the surface profile measurement of freeform surface.