The emergence of optical clock and the precision limits of the existing time synchronization space-based make fiber one of the major high-precision time service systems. A Round-Trip method that uses dense wavelength division multiplexing (DWDM) technology in a single fiber and achieves bidirectional time information transfer is introduced. This method restrains the delay time asymmetry of the optical time service using fiber business channel. The time deviation and random error are analysed by using typical parameters of various modules and the character of the fiber channel in the system,and the corresponding suppression of deviation and compensation of random error methods are brought forward. Theoretical analysis shows that the method can satisfy time-precision of nanosecond order of magnitude in the existing fiber distance relay. Taking advantage of the time interval,the delay time and the time stability of the major modules in the 50 km fiber time service system are measured,verifying the theoretical analysis.