The tool point frequency response function(FRF) is commonly obtained by impacting test or semi-analytical techniques.Regardless of the approach,it is assumed that the workpiece system is rigid.The assumption is valid in common machining,but it doesn’t work well in the cutting processes of thin-wall products.In order to solve the problem,a multi-degree-of-freedom dynamic model is employed to obtain the relative dynamic stiffness between the cutting tool and the workpiece system.The relative direct and cross FRFs between the cutting tool and workpiece system are achieved by relative excitation experiment,and compared with the tool point FRFs at x and y axial direction.The comparison results indicate that the relative excitation method could be used to obtain the relative dynamic compliance of machine-tool-workpiece system more actually and precisely.Based on the more precise relative FRFs,four evaluation criterions of dynamic stiffness are proposed,and the variation trend curves of these criterions during the last six months are achieved and analyzed.The analysis results show that the lowest natural frequency,the maximum and the average dynamic compliances at x axial direction deteriorate more quickly than that at y axial direction.Therefore,the main cutting direction and the large-size direction of workpieces should be arranged at y axial direction to slow down the deterioration of the dynamic stiffness of machining centers.The compliance of workpiece system is considered,which can help master the deterioration rules of the dynamic stiffness of machining centers,and enhance the reliability of machine centers and the consistency of machining processes. The tool point frequency response function (FRF) is commonly obtained by impacting test or semi-analytical techniques. Regardless of the approach, it is assumed that the workpiece system is rigid. The assumption is valid in common machining, but it does not work well in the cutting processes of thin-wall products. order to solve the problem, a multi-degree-of-freedom dynamic model is employed to obtain the relative dynamic stiffness between the cutting tool and the workpiece system. The relative direct and cross FRFs between the cutting tool and the workpiece system are achieved by relative excitation experiment, and compared with the tool point FRFs at x and y axial direction. The comparison results indicate that the relative excitation method could be used to obtain the relative dynamic compliance of machine- tool-workpiece system more actually and precisely. based on the more precise relative FRFs, four evaluation criterions of dynamic stiffness are proposed, and the variation trend curves of these criterions during the last six months are achieved and analyzed. the analysis results show that the lowest natural frequency, the maximum and the average dynamic compliances at x axial direction deteriorate more quickly than that at y axial direction. wherefore, the main cutting direction and the large-size direction of workpieces should be arranged at y axial direction to slow down the deterioration of the dynamic stiffness of machining centers. The compliance of workpiece systems is considered, which can help master the deterioration rules of the dynamic stiffness of machining centers, and enhance the reliability of machine centers and the consistency of machining processes.