The interim guidelines and explanatory notes of the Second Generation Intact Stability Criteria (SGISC) have been formally approved. As one of the five failure modes in the SGISC, pure loss of stability is a nonlinear dynamic phenomenon accompanied by large-amplitude roll motions. To achieve the prediction of pure loss of stability in regular waves, this study develops a 6-DOF time domain weakly nonlinear model coupling maneuvering and seakeeping motions. The roll restoring variation induced by wave is crucial for the occurrence of pure loss of stability. Meanwhile, the hydrodynamic lift force generated by hull heel significantly impacts the accurate prediction of roll restoring variation. The ONR tumblehome is chosen as the research subject. The hydrodynamic lift force is computed via the Vortex Lattice Method (VLM), in which pressure differences on port and starboard sides of the hull are integrated. This lift force is then incorporated into the 6-DOF mathematical model to predict capsizing behavior in following and stern quartering waves. Numerical simulations are systematically compared with experimental results, demonstrating that the 6-DOF mathematical model incorporating the hydrodynamic lift force can be utilized to predict the capsizing range of pure loss of stability.