Abstract: In order to improve the positioning accuracy of short-base-line underwater acoustic positioning systems,a two-step algorithm for estimating time-delay difference value is analyzed theoretically and verified by simulation and experiments,and the implementing procedures are presented.In the algorithm,time-delay difference value is regards as combination of the value of initial time-delay difference testing and the value of time-delay difference modifying. The former is obtained from the peak value of envelopes of correlation function;the latter is calculated by phase difference estimation in frequency domain,because mapping exits between time-delay difference modifying value and phase difference value.The simulation and experimental results show that this method achieves high accuracy for time-delay difference estimation.The two-step algorithm is practical and can improve the positioning accuracy of shortbase-line underwater acoustic positioning systems effectively.