The mechanical strength of immobilized beads was enhanced using sodium alginate as the embedding agent and CaCl2 as the crosslinking agent. Orthogoal experiments and one-factor experiments were designed to optimize the immobilization conditions. The effects of quantity of entrapmenting bacteria, concentration of sodium alginate, crosslinking time, and diameter of the immobilized beads on the immobilization were studied. Under optimum conditions, the nitrogen removal ability of immobilized bacteria was similar to that of dissociation bacteria.