Time series of the porewater pressure in seabed under the action of the changing water pressure was experimentally investigated. Firstly, the response of porewater pressure to an abrupt change of the water pressure at the bed surface was measured. Secondly, a simple impulse-response model was proposed to estimate the time scale of the relaxation of porewater pressure on the basis of the experimental results. Finally, the transient process of porewater pressure under a sinusoidal change of the water pressure at the bed surface was reproduced by the proposed impulse-response model.