In order to solve the problem of low signal-noise ratio(SNR)of raw data for optical fiber sensing cable external damage monitoring,in this paper,a hybrid signal-processing method is proposed based on an improved spectral subtraction denoising algorithm combined with detrending techniques from time-domain data by denoising in the time domain based on the improved spectral subtraction method.The method implement the extracted baseline of optical power attenuation to remove the attenuation trend in the spatial domain of the original data and to eliminate the influence of optical power on signal amplitude,which caused by distance attenuation.The effectiveness of proposed method was tested by the on-site construction signal.The results verified that both detrending and improved spectral subtraction can improve the signal-noise ratio(SNR),and both have a superimposing effect.And the signal-noise ratio(SNR)after signal-processing of the on-site crusher,excavator and pile driver was increased by 2.25 dB,6.11 dB and 4.72 dB respectively.This method has a certain reference value for follow-up research on the location and identification of monitoring potential external damage of optical fiber sensing cable.