The full-vector finite element method is applied to investigate the property of a photonic crystal waveguide coupler consisting of a square lattice of dielectric cylinders in air. The coupling ratio with different coupling lengths, wavelengths and refractive indexes are given. The simulation results show that the photonic crystal waveguide coupler (PCWC) can be widely used as switches, multiplexers and demultiplexers by changing certain structure parameters and coupling ratio.