The measurement system for laser tracker’s target-missing quantity based on four-quadrant detectorsis introduced. According to the operation principle of the detector and the facula characteristic of tracking laser, the re.lationship between target-missing quantity and the output current of the four-quadrant detector is nonlinear. Accord.ing to the problem, piecewise linear interpolated algorithm is adopted. The measurement system is calibrated by ahigh-precision displacement platform and measurement experiments are performed. The results show that the systemhas a fast measurement speed and high measurement accuracy. The time of measurement per second is up to 600times or more and the accuracy is 5 m within ±500 m measurement range. The system can be widely applied in the re.lated fields.