Based on optical switch and fiber, the topologic structure of optical fiber delay line was designed. The influence of errors caused by optical fiber connection and the optical switch on precision of optical fiber delay line was analyzed. In order to design the optical fiber delay line of high precision, the method was found to reduce the error caused by optical switch. Optimal topologic structure was adopted, the software MATLAB was used to figure out all the results and the best structure would be found. The simulation result shows that, when the error of optical switch is less than 1 ps, the precision of optical fiber delay line is below 2 ps. Magneto optical switches and single-mode fiber were used to build 5bit optical fiber delay line. Experimental measuring results show that, when the step length is 100ps, the precision of the fiber delay line is better than 2.98 ps.