Abstract: The velocity parameter is an important parameter for the passive ranging of three-element array. When the sonar works at high frequency, the velocity parameter is usually set as the sound speed of water. With the sonar technology developing for the long range detection, sonar frequency is becoming much lower. Phase velocity and group velocity of low-frequency sound in shallow water are completely different from the sound speed of water. So, it is important for getting a higher ranging accuracy to determine which velocity should be chosen for passive ranging of three-element array. Theoretical analysis and simulation results in Pekeris waveguide show that the phase velocity should be used for passive ranging of three-element array at low frequency in shallow water, and it could improve the ranging accuracy effectively.