The model of a beam propagating in a high power laser system is built based on relay imaging. The displacement sensitivity of the lens to beam positioning error is obtained using this model, which is then compared with the traditional method. Two real systems, the pre–amplifier and four–pass amplifier in SGII-U, are presented to further discuss the differences between the two methods. The limitation and application range are summarized in the end. The findings can be used to provide guidance in similar systems.