A novel method to fabricate twodimensional photonic crystal alloptical switch is reported. Photonic crystal waveguides can be obtained by infiltration of azobenzenedoped liquid crystals into air holes in twodimensional photonic crystals composed of hexagonal lattices of air cylinders. Numerical simulations show that the photonic crystal band gaps are modulated by nematic liquid crystals infiltrated in the air holes. Then the band gap can be controlled easily under the influence of the external optic field. So the results can serve as an alloptical switch. Compared with the tunable method through an external electric field, the technique has the advantages of fast response and low cost.