Sodium alginate (SA) was modified for better water resistance with potassium persulfate (as the initiator) and acrylonitrile (as the monomer) via free radical graft copolymerization, and the corresponding sodium alginate-acrylonitrile graft copolymer film (SA-g-AN) was prepared. The modified SA film was characterized by measuring contact angle and water solubility and with DSC. The copolymerization mechanism and the microstructure of the graft copolymer were studied by FT-IR, ¹H NMR, TEM and SEM. The correlated properties of the SA-g-AN film materials were measured with AFM, TG, XRD and DMA. The results indicated that the pyran ring of SA was open during the reaction, the morphology of the product exhibited a solid spherical cluster, and the density of the graft copolymer decreased as the content of acrylonitrile increased. The water resistance was improved after SA was modified by acrylonitrile. Both its crystallinity and thermal stability improved by introducing the acrylonitrile unit. The storage and loss modulus were in between those of SA and PAN. The SA-g-AN materials had a better film formation property and an excellent adsorbability towards heavy metal ions. These results have proven that this is a better development prospect on the field of sewage treatment.