A phasesensitive optical timedomain reflectometer(更-OTDR)system based on improved compressed sensing technology is studied.For compressed sensing technology,signal sparsity K directly affects the quality of signal recovery.In order to improve thequality of signal recovery,K value that is closer to the real sparsity of the original signal should be obtained.Therefore,a method of calculating signal sparsity K based on hard threshold is proposed in this paper.At the same time,the reconstruction method combined with contraction threshold is used to preserve the effective components of the signal as much as possible.The experimental results show that the sinusoidal vibration signal with vibration frequency of 1 kHz is reconstructed successfully at a distance of 5.6 km.Compared with the common hard threshold compression sensing technology,moving average denoising technology and Sym8 wavelet denoising technology,the SNR of compressed sensing technology in this paper is improved by 3.55,13.2 and 8.02 dB, respectively.