Based on the optical switch and fiber, the topological structure of optical fiber delay line(OFDL) was designed, two aspects were analyzed which have influence in precision of optical fiber delay line, that is the error of connection and the error caused by optical switch. In order to realize high precision of optical fiber delay line, a method was found to reduce the error caused by optical switch, by simulating, when the error caused by an optical switch is 1 ps, the precision of optical fiber delay line can reach 2 ps, and this method can be applied to any bit OFDL with arbitrary optical switches. Magneto optical switches and fiber were used to build a system of 5 bit optical fiber delay line. The optimal topologic structure was adopted and the precision of the fiber delay line was measured up to 2.85 ps.