In this paper, the mechanism of detonation to quasi-detonation transition was theoretically discussed, a new physical model to simulate quasi-detonation was proposed, and one-dimensional numerical simulation was conducted. This study first demonstrates that the quasi-detonation is of thermal choking. If the conditions of thermal choking are satisfied by the chemical release of energy and some disturbances, the supersonic flow is then unable to accept the additional thermal energy, and the CJ detonation becomes the unstable quasi-detonation precipitately. The kinetic energy loss consumed by this transition process is first considered in this new physical model. The numerical results are in good agreement with previous experimental observations qualitatively, which demonstrates that the quasi-detonation model is physically correct and the study is fundamentally important for detonation and supersonic combustion research.