Aiming at the lack of more in?depth quantitative data for the study of the systematic errors of Littrow?type grating interferometers, the systematic errors of Littrow?type grating interferometers caused by the positioning accuracy of the components, namely, the systematic errors of the interferometers caused by the additional optical path differences due to grating rotation around the x, y, and z axes as well as mirror rotation around the y axis, were investigated in terms of the impact of the systematic errors on the displacement measurements of the interferometers. A mathematical model of the error caused by the change in optical path difference when the grating and mirror rotate around the axes was established, quantitatively analyzed, and the accuracy of the mathematical model was verified by experiments. The results show that: when the grating and mirror rotate around the x and z axes, no additional optical path difference is generated; when the grating rotates around the y axis, the systematic error will be generated and increase with the increase of the grating constant and the rotation angle; when the mirror rotates around the y axis, the error will be generated only when the rotation angles of the two mirrors are different, and the error will increase with the increase of the rotation angle of the two mirrors. After synthesizing the errors of the whole system, the undefined system error is ± 3.12 μm in high assembly level, and ± 17.75 μm in general level, which verifies the correctness of the theoretical simulation, and provides technical reference and theoretical support for the system design of the Littrow?type grating interferometer.