This paper described the absorbency characteristics of starch grafted poly (acrylamide) in saline solution. The parameters affecting water absorption such as ionic type, concentration of external solution, were investigated respectively. It is proved that water absorbency depends largely on the cationic type and concentration of the solutions in terms of ionic strength, irrelative to anionic valence and type. The results show that the process of water absorption is consistent with Donnan equation. The swelling behavior is related to the osmotic pressure of the hydrogel, which acts as a polymer solution confined inside a semi-permeable membrane.