The seepage numerical models of typical sections of cofferdam of Honggu Tunnel are established; and cofferdam seepage properties under 11 kinds of construction conditions when the permeability coefficient k and thickness d of the cutoff wall and the weir head hr varying, are analyzed. The results indicate that: 1) The cofferdam seepage flow per unit width Q reduces with the permeability coefficient k decreases; and the variation velocity of Q reduces gradually; the peak of hydraulic grade Js remains stable in cutoff wall. 2) The Q and Js reduce with the cutoff wall thickness d decreases; and the variation velocities of Q and Js reduce gradually. 3) The Q and Js reduce linearly with the weir head hr decreases. The design values of k and d with good antiseepage ability and economic benefits are obtained; furthermore the mixing proportions of corresponding plastic concrete which can meet the design requirements are presented. The antiseepage performance of plastic concrete wall is detected and evaluated by high density resistivity method and monitoring of water levels inside and outside cofferdam.