With the aim to resolve the influence of the performance degeneration of FBG sensors on multiplexed demodulation, the affects of reflection sidelobes on the demodulation of a series multiplexing FBG sensor was analyzed based on the demodulation theory of FBG strain sensors. When the performance of the FBG sensors was degenerated, it was found that the bandwidth of the reflection spectrum was broadened, the peak value fell, and the reflection sidelobe was augmented by both theoretical deduction and simulation. The effects of reflection sidelobes and the link-connected loss on the FBG multiplexed demodulation were analyzed in detail, then the floating demodulation threshold and suppressing reflection sidelobe were presented to resolve the effect and to reduce the error rate of the multiplexing demodulation. Finally, the theory is validated and the method is demonstrated experimentally. Experimental results show that the error rate of the multiplexing demodulation reduces from 11.36% to 0, and the accuracy of the measurement result is greatly improved.