X－ray diffraction ( XRD) is an important tool for analyzing the structure of substances and has been paid more and more attention in the teaching of physics － related majors． In this paper，the measurement errors caused by the variation of the sample stage height Δz are systematically analyzed． It is found that the sample stage height shift Δz causes a significant deviation in the diffraction peak position Δ2θ，which in turn causes a deviation in the calculation of the crystal interplanar spacing d and the lattice constant a． The results of the data fitting show that the deviation of Δd /d and the deviation of Δa /a for Si are approximately linear with Δz． Combined with the Bragg－Brentano diffraction geometry，we have theoretically analyzed the above experimental results． This work pro-vides a theoretical basis for the accurate determination of lattice constants in XRD experiments，and will also pro-vide an important reference for the development of related teaching work．