Considering the influence of boundary conditions, and in order to further improve the identification accuracy and calculation efficiency, a method for dynamic identification and stability analysis of slender parts turning based on flexible boundary constraints was proposed. A slender workpiece model with flexible boundary constraints was established to improve the prediction accuracy of turning stability. In this model, the 2D flexibility of tools and parts and the influence of tool tip radius were included in the dynamic force modeling, and the dynamic characteristics of slender workpieces in the chuck tailstock dynamic system were calculated according to the continuum theory. Then, the optimal connection stiffness was determined by minimizing the deviation between the experimental value and the simulation value of the characteristic parameters. The experimental results show that the proposed method significantly improves the steering prediction accuracy and stability limit.