Abstract: In order to solve the problem that the squareness error of the guide block pair measured by the traditional autocollimator is inconsistent due to different measuring objects, a general method for calculating the squareness error is proposed. Firstly, the two guide rail slide pairs were considered separately, and the straightness error of the guide rail was measured and analyzed according to the measuring principle of autocollimator and the envelope model of minimum inclusion area. Secondly, the data analysis and processing procedure was programmed in Matlab. Finally, the experiment was carried out on a small CNC milling machine produced by a certain company. The results show that the measured values are within the tolerance range of the machine tool accuracy test, which proves the validity of the calculation method, and provides a more general calculation method for the measurement of squareness error between guide rails.