Abstract: As propagation velocity of acoustic wave in rocks is closely related with rock properties, porosity and properties of fluids in pores, acoustic travel time anomaly in fault rock can be used to determine the dynamic properties of faults. Studying on variations of the acoustic travel times of sandstone and mudstone and the fault rocks with depth in Dongying depression indicates that variation amplitude and scope of acoustic travel time anomaly of exten-sional faults are related with burial depth of the faults. When the burial depth is less than about 1600m, the a-coustic travel time in fault zone is larger than that in surrounding rocks and the faults are open faults without sealing ability. While when the burial depth is larger than 1600m, the acoustic travel time in fault zone is less than that in the surrounding rocks and the faults are closed faults of compressional nature that can act as seals.