A copolymer hydrogel for immobilized microorganism carrier was synthesized by using sodium alginate（SA），polyvinyl alcohol（PVA）and active carbon as start materials and calcium chloride and boric acid as crosslinker.The ionic diffusion mechanism was analyzed and the diffusibility of NH+4，NO-2 and NO-3 was measured by the static penetration method.The result showed that the hydrogel prepared by crosslink 4% PVA，2.0% sodium alginate and 0.2% active carbon with 5% calcium chloride and saturated boric acid as crosslinker for 15 min possessed the highest ionic permeability.The degree of equilibrium swelling and ionic diffusion coefficient of the composite hydrogel increased with increasing concentration of PVA.The degree of equilibrium swelling decreased with increasing concentration of SA but the ionic diffusion coefficient first increased and then decreased with increasing SA concentration.The diffusion rate of the inorganic nitrogen-containing ion in the hydrogel decreased as fellows：NH+4> NO-2> NO-3.The synthesis condition and the start materials ratio had a great impact on the diffusion coefficient of NO-2 and NH+4，but a relatively small impact on that of NO-3.