Photoelectric composite cable is the lifeline of tethered unmanned aerial vehicle, which transmits power and signal. When the internal tension of the cable is too large, the internal optical fiber will break. In order to monitor the tension distribution inside the cable, it is necessary to measure the strain of the cable. When fiber Bragg grating sensor is used to measure the axial strain of the composite cable, it is found that the large-scale and rapid movement of the slender composite cable will easily cause the local bending of the cable, resulting in the splitting of the reflection peak of the reflection spectrum. Based on the theory of mechanics of materials, the reason why the reflection peak splits during the bending of composite cable is analyzed. The theoretical analysis result is verified by experimental, and an improved measurement method is proposed, which avoids the influence of viscose and internal shear force of cable on the measurement result, and effectively improves the accuracy of axial strain measurement.