This paper provides an optical fiber time synchronization technology based on the double fiber round-trip ratio method. The system can directly measure the unidirectional transmission delay from a master station to a slave station through the measured values of three time interval counters and the correlation ratio relationship. This method eliminates the influences of fiber length expansion and round-trip delay caused by environmental changes on the time synchronization accuracy. The master and slave stations are connected by 100 km and 75 km optical fibers, respectively. When the fiber link temperature changes from -20 ℃ to 40 ℃ and the wavelengths of 1490 nm and 1550 nm are used, the estimation error of unidirectional delay can be reduced to 260 ps, approximately. When 1310 nm and 1550 nm wavelength pair is used, the timing accuracy is improved about 1.3 ns.