Abstract: Insufficient stiffness of CNC machine tools is the root cause of cutting vibration, which in turn affects tool life, machining quality and cutting efficiency. Based on the testing and analysis technology of the dynamic characteristics of the whole machine tool, this paper proposes a set of technologies that can effectively solve cutting vibration of CNC machine tools and diagnose weak structural stiffness. The core of this technology lies in the testing of the whole-machine dynamic characteristics using the Bayesian operational modal analysis method. This method takes the ambient vibration during machine tool operation as excitation, enabling one-time, high-precision acquisition of the whole-machine modal parameters and overcoming the shortcomings of traditional methods. Through the systematic workflow of "modal testing-vibration spectrum analysis-frequency and mode shape comparison", the specific weak-stiffness components causing vibration can be accurately identified. Taking a certain gear grinding machine as a case study, this method successfully diagnosed stiffness defects in its column, base and other components. Practical results demonstrate that this technology has been implemented in the diagnosis and optimization of over 40 CNC machine tools of different types. It offers direct quantitative evidence for machine structural optimization, enhances machining stability and cutting efficiency, and effectively mitigates vibrations in cutting processes.