In this study, according to the seepage characteristics of backfill behind a quay wall on the highly permeable foundation soil, a differential program for the onedimension transient seepage was built based on the seepage theory of saturated and unsaturated soil under the condition of waterlevel rapid drawdown. The reliability of the program was verified by the model test and the numerical simulation. Then the program was used to study the influences of the drawdown speed, the permeability coefficient and the water content of saturated soil on the change of water pressure during the seepage. The results show that the drawdown speed and permeability coefficient have a great influence on the change of water pressure during the rapid drawdown of water level, while the water content of saturated soil has negligible influence.