In order to improve the performance of dual Mach-Zehnder interferometer (DMZI) distributed optical fiber sensing system,a positioning algorithm based on signal derivative analysis is proposed in this paper.By means of the derivative value at the zero level of the signal,the algorithm first extracts h igh frequency part from the original signal and then delay estimation is carried out for this part of signal to get t he invasion event location.Since the frequency and bandwidth of the extracted signal are relatively large,the pos itioning error of the system is small.Compared with extracting the signal of starting point by discre te wavelet,the algorithm can adapt to more vibration event types and due to the computation of derivative is much l ower than the Fourier transform,the computation time is significantly reduced and the real-time performance is impr oved.Experimental results show that by using sampling rate of 10M/s,the algorithm has a low positioning error of 20m and th e processing time is less than 0.13s.The performance of the system is improved.