Abstract: Anti-seepage and reinforcement for dikes and dams have been a huge and heavy task in China. The current technologies of dikes and dams using vertical impermeable and reinforced structures have disadvantages and limitation and become a bottleneck in design of practical seepage-proof and reinforcing works. In contrast, non-water reacted two-component polyurethane has a good overall performance and environmentally friendly properties; it is suitable for seepage-proof and reinforcing material of dikes and dams. This paper describes a new method for constructing polymer flexible anti-seepage walls that is based on the material properties and the mechanism of diffusion in soils. Compared with the conventional anti-seepage walls of cement concrete, this method has advantages of little disturbance, good anti-seismic, anti-cracking property, and convenience in construction. We also present three corresponding technologies of polymer grouting ? underwater grouting, membrane bag grouting, and compound grouting ? which are based on the characteristics of polymer reaction and expansion in water and the mechanism of polymer diffusion in coarse-grained soils. All these grouting technologies are particularly suitable for treating localized leakage and piping in water conservancy projects. Polymer grouting technologies for earth dikes and dams have been applied in a variety of major projects and their seepage-proof and reinforcing effects are excellent.