Two photonic crystal waveguides with triangular lattice of air holes together,between which there are three rows of air holes,are put in parallel to construct a directional coupler.The propagation of TE(electric field parallel to the axis of the air holes) and TM(magnetic field parallel to the axis of the air holes) polarizing lights in the proposed directional coupler is analyzed numerically.The results show that the coupling length of TE light is reduced whereas the coupling length of TM light do not change when the radius of one row of the dielectric rods between the two waveguide of the coupler are modified.On the basis of this structure,ultracompact polarizing beam splitter is designed and the length of the device is only 10.1 μm.Compared with the previous results of 24.2 μm,this device is smaller and more efficient.