A non-contact three coordinate measuring system was established to implement the measurement of 3D free-form surfaces accurately.A laser sensor was installed on the Z axis of a coordinate measuring machine via a probe head with two rotary axes,whose direction could be adjusted according to the shape of the measured surface.For the purpose of enabling the sensor to realize measurement functions in every orientations,a calibration method of laser beam direction based on a spherical target was proposed,and its principle was analyzed in detail.In the calibration procedure,the sensor moved in an equal distance along X,Y and Z axes and then equation sets were set up based on the bundle equations to compute the unit direction vector of the laser beam.Finally,a cube block with known dimensions was measured by the system to test the repeatability of the measuring system.The experimental result indicates that the measuring uncertainty of the system is 0.048 mm,which shows the good practicability of the calibration method.When the system was used to measure a sphere with a known diameter,the errors in every orientations are all smaller than 0.05 mm,which manifests that the calibration method proposed meets the requirements of reverse engineering.The method shows higher calibration precision and repeatability,and lays a basis for fast scanning of 3D free-form surfaces.