CNC lathe spindle box is a key component of the CNC lathe system, and its dynamic characteristics are the main factor affecting the machining accuracy and processing efficiency of machine tools. In this paper, CK61125 CNC lathe spindle box model was established by simplifying its structure. Then modal analysis of the spindle box was done and extended to 6-order modes with the platform of co-simulation environment in ANSYS Workbench, and the natural frequencies and vibration modes for lathe spindle box were obtained. The weak links in the spindle box were found, and the improvement schemes are proposed. Finally the lathe spindle box was improved, the modal analysis of the spindle box was made again, and the results showed that the lathe spindle box dynamic performance was improved. All of these results are beneficial for optimization and re-design of the lathe spindle box so that it has better dynamic performance.