Plastic rectangular phase grating wavelength division multiplexing (WDM) device can effectively use plastic optical fiber bandwidth resources and improve the effective bandwidth of metro access network. The formula of the negative first-order diffraction efficiency is derived by using rigorous coupled-wave analysis in the optimum incident angle and the grating effective modal index varying with grating period is numerically simulated. Simulation results indicate that when the rectangular phase grating with refractive index of 1.58 and duty cycle of 0.5 is illuminated by 1.5 μm light, the negative first-order diffraction efficiencies for both TE and TM polarized waves can reach the maximum simultaneously under the conditions of the optimum grating period of 0.82 μm and the minimum groove depth of 1.47 μm. The normalized diffraction optical intensity is measured for different incident angles. The experiment result shows that, with the optimum incidence, the polarization dependence loss of the plastic rectangular phase grating can be lower than 0.5 dB.