Due to the limited precision of the existing space-based time synchronization network and the emergence of optical clock, using fiber to transfer and detect time signal has become a major high-precision time-synchronization system. The principle of high precision time transfer is introduced, the sources of error in time transfer system using optical fiber are analyzed and some methods to improve the system are proposed. Meanwhile, a new coding and decoding mixed with analog and digital methods is used to design and outcome a time transfer system with optical fiber line. Finally, the experiment of optical time transfer system is carried out and the time accuracy is tested, which is approximately 200 ps while the time veracity is ±600 ps. These result are better than any existing space-based time transfer system.