A measurement system of coaxiality for long-span holes is developed based on position sensitive detector（PSD） and laser collimation.Measured section is determined by self-centering mechanism in holes of multifunctional probe

and the center coordinates of each hole’s section are determined after laser spot position is detected by PSD at the front end of probe

by which the coaxiality error can be evaluated.Mathematical model of laser self collimation is deduced and according to the model

the laser position can be adjusted with a four-degree-of-freedom platform.The experiments of laser collimation as well as the measurement of coaxiality are performed.Experimental result shows that the deviation between the laser beam and the center line of both end holes of the workpiece is within ±0.032 mm and the relative error of coaxiality error measurement system is 8 % compared with three coordinate measuring machine

and has high precision of collimation and coaxiality error measurement.