The optical path part is the main comp onent of the fiber delay line si gnal delay.In order to reduce optical loss、improve system performance and opti mize composition structure, a design scheme of the optical fiber sub-system fo r X-band optical fiber delay line is proposed.First,we designed the overall s t ructure of the system.Then,based on the expansion effect of the fiber and the improved Marcuse D formula,we built a bending radius model of the fiber and a nalyzed it.Then according to the calculation results、fiber length and the actu al production accuracy,the main structure of the optical path is designed and realized.And,theoretical analysis and calculation of the fiber transmission c haracteristics are did.Finally,we put the optical fiber sub-system into over a ll structure to conduct testing and analysis for transmission performance of the system by using a vector network analyzer.The results show that the insertion loss of the optical fiber is between －38.56dB to －39dB,the amplitude flatn ess ≤±1.5dB,The change in gain during temperature drift is less than ±1dB.Ther efore,the system performance is stable and reliable.It basically meet the req uirements of signal transmission index and error range for optical fiber delay l ine.