A novel method for measuring four-degree-of-freedom geometric errors of a linear stage is put forward. A telescope is used to magnify the straightness errors and angular errors in many times. Position sensitive detector (PSD) is used to sense the errors very fast. So the horizontal and vertical straightness errors as well as yaw and pitch errors can be measured simultaneously with high precision. The four-degree-of-freedom measuring system has been developed,and the theory of measurement has been analyzed. The calibration,stability and repeatability experiments are completed. Experimental results and theoretical analyses show that the system has the resolution of less than 0.2 μm for straightness error measurement and 0.3″ for angular error measurement,and the measurement accuracies of straightness error and angular errors are about ±0.7 μm/cm and ±0.3″/cm respectively within the measurement range of 10 cm.