Abstract: Precision retention is considered one of the key performance indicators of CNC machine tools. Currently research on the precision retention of CNC machine tools primarily focuses on the analysis of precision values at specific moments or over different time periods. Relatively few studies address the fluctuation of degradation trajectories in relation to time variation. Based on the theory of multi-body systems, a spatial position error model is established for CNC machine tools in this paper, and spatial position precision degradation is extracted as the core degradation indicator reflecting the overall precision retention of the machine. Using the radial basis function method, the degradation data obtained from periodic measurements are transformed into a continuous degradation trajectory model. The fluctuation degree of the degradation trajectory is analyzed, and the concepts of average degradation rate and precision fluctuation degree are introduced. This enables the effective quantitative evaluation of the CNC machine tool's precision retention. The modeling and evaluation method proposed in this study allows the accurate assessment of the impact of degradation trajectory fluctuations on precision retention and provides a theoretical basis for the preventive maintenance of CNC machine tools.