Abstract: The horizontal vibration of the soil is solved by potential functions based on the theory of wave propagation by supposing the soil around the pile satisfy the Novak plane strain model, and the rigorous expression of the attenuation function of the horizontal displacement of the soil caused by the vibration of the pile is obtained. The lateral dynamic stiffness and damping of the soil are also attained. The pile-to-pile dynamic interaction and the horizontal dynamic impedance of pile groups in soil were investigated by initial-parameter method, and were compared with those of the approximate plane strain model proposed by Gazetas and Dobry. The result indicated that the difference between the result of a rigorous plain-strain model and an approximate model is related to the pile space and the angle between the lines of two piles as well as the direction of the horizontal applied force, and that the pile space has great influence on the pile-to-pile dynamic interaction and the horizontal dynamic impedance of pile groups in soil.