Abstract: The pure loss of stability of a ship is one of the failure modes of the second-generation intact stability criteria. Based on the static surface coordinate system, the generalized pitch angle and draught variable are defined, and a wave equation with clear physical meaning is deduced. Based on the Froude-Krylov hypothesis, combined with AutoCAD 2D surface area computing technology and the VBA programming method, a calculation method of the loss of pure stability of ships in regular waves is proposed. For a naval ship, the righting arm is calculated in regular waves, and the large heel ship state is shown to have an identical convergence. The results of the calculations show that a significant reduction in the meta centric height of the maximum righting arm occurs not just in a wave crest but also in a trough. By analyzing the wave profile under the hull, it can clearly be seen that wave amplitude above the deck or below the bottom of the hull causes the pure loss of stability, and in oblique waves or beam seas, the pure loss of stability is caused the asymmetry of the wave profile on the hull. The coinciding convergence of the calculations shows that the process of the definition of generalized pitch can be employed to assess the pure loss of stability of naval ships in regular waves.