In order to ensure the data security of a fiber-terahertz seamless integrated communication system, this paper proposes a dynamic one-time-pad (OTP) terahertz (THz) -coherent passive optical network (PON) seamless integrated physical layer encryption scheme based on double chaotic models, which utilizes the ultra-high sensitivity of the chaotic model to realize the real sense of OTP in the digital domain. The chaotic sequence generated by auxiliary chaos is mapped to selection factors to control the main chaotic system so as to carry out a dynamic one-time selection of keys, and the obtained dynamic chaotic sequence is interleaving and regenerated to encrypt the original data. It is worth noting that the key corresponding to each original encrypted data is different, which can realize the dynamic OTP of the original data. In this paper, we successfully demonstrate the seamless integrated encryption transmission of THz-coherent PON based on optical frequency combs (OFC) that the 30 GBaud dual polarization 16 quadrature amplitude modulation (DP 16QAM) signals are successfully demodulated over a span of 25 km standard single-mode fiber (SSMF) and 10 m wireless link at 130 GHz and 150 GHz. The number of dynamic key updates reaches 8 × 1012. Compared with the typical three-dimensional Lorentz chaotic model, the initial value sensitivity of the chaotic model is 2 times, the sensitivity of the encryption system is more than E-14, and the key space is up to 8 × 10228. The performance of the encryption scheme at different baud rates is verified by experiments, and the feasibility of the scheme is further demonstrated. The experimental results show that the scheme has great application potential in future fiber-THz seamless integrated secure communication.