Seepage is ubiquitous during excavation in high groundwater table. Analytical formulas for calculating the water head in active and passive zones of foundation pit are derived under the assumption that the seepage is one-dimensional and steady. The influence of seepage on the effective stress and the heave of foundation pit base is analyzed through a numerical example. It is indicated that seepage causes the decrease of effective stress and thus the increase of heave of soil mass beneath foundation pit base.