In order to realize the precise measurement of the parallelism error, the laser collimating method of parallelism error measuring method based on position sensitive device (PSD) was proposed, and the experimental measurement system was designed. The divergence angle and spot size of the alignment laser beam were balanced by using the method of the second lens transformation of the inverted telescope structure. The optical path was turned by using an optical pentaprism, and then the position of a point relative to the measurement reference can be obtained in real time by PSD signal conditioning circuit as well as data acquisition and processing system. Then, the parallelism error between the toleranced feature and the datum feature can be quickly evaluated by using the minimum zone method, and the theoretical analysis and experimental verification were carried out. The experimental results show that the relative uncertainty of the measuring system is 0.077%, which has high measurement accuracy. It provides an effective measurement method for the precision measurement technology of parallelism error and has certain practical guiding significance.