Two-way fiber-optic time transfer (TWFTT) is a promising precise time synchronization technique with subnanosecond stability. However, the asymmetric delay attack is a severe threat, which can deteriorate the performance of the TWFTT system. In this article, a clock model-based scheme is used to defend the subnanosecond asymmetric delay attack. For the scheme, a security threshold is set according to a two-state clock model, and the estimated frequency difference is excluded from the measured time difference to detect the subnanosecond asymmetric delay attack. Systematic detection and mitigation scheme for asymmetric delay attack is developed in this article. Theoretical simulation and experimental demonstration are implemented to explore the feasibility of the method. A TWFTT system of time stability with 24.5, 3.98, and