A passive sensing system using piezoelectric sensors was developed for realtime monitoring and estimating the external lowvelocity impact on composite structures.The system included distribute piezoelectric sensor network,portable acquisition hardware and data analysis software.Piezoelectric sensor network covered the focus areas of the structure,and realtime in situ monitoring the stress waves generated by the impact. Multichannel acquisition hardware could parallel acquisition and recorded the sensor signals and exchange data with analysis software using direct memory access (DMA) technique. For evaluation the impact events,an improved systemidentification method was integrated in the analysis software.Combining the integrated systemidentification model and the obtained realtime sensor data,the analysis software would estimate the impact location and perform the inverse reconstruction of the impact force.Testing results showed that the developed passive sensing system could exactly identify the impact events on composite structures, and estimated the location and force time history effectively.