The water absorption principle of water retaining agent is the same as the general SAP. It is formed by the expansion of tension (which is produced in water the polymer electrolyte molecule chain amide base and/or carboxyl group) and the expansion of tension (which is produced because of the restriction molecular chain of intersection point) interacting each other. Polyacrylamide, for example, there are a large number of amide and carboxyl hydrophilic group in the water retaining agent. By osmotic pressure (which is produced by concentration difference between resin internal ion, base group and the aqueous solution related components) and the affinity (which is produced by polymer electrolytes and water), a lot of water can be absorbed until the concentration difference disappeared. However, rubber spring can control water retaining agent to achieve satisfactory water absorption level. The higher crosslinking degree of the molecular structure is , the stronger the rubber elastic force is. And the equilibrium point of the rubber elastic force and the suction force is its apparent absorption capacity. Water retaining agent does not dissolve in water, but it can absorb water which is quite into one hundred times its weight. Water retaining agent can effectively inhibit the water evaporation. After the water retaining agent is infiltrated in soil, to a great extent, the water evaporation is suppressed, the saturated water content of soil is increased, and the saturated water conductivity of soil is reduced. So, it can slow down the speed of soil releasing water, reduce the loss of the soil water infiltration and achieve the goal of catching water. It can also stimulate crop root growth and development, increase the root length and the root number. Finally make crop keeping good growth under drought condition.