The diffraction anomalies of a one-dimensional TiO₂ subwavelength grating (SWG) are studied, which are characterized by leakage mode resonance effect and Rayleigh anomalies. The results show that Rayleigh anomalies and leakage mode resonance effect occurrs for transverse magnetic wave (TM polarization) and transverse eletric wave (TE polarization) incident under certain parameter conditions. In the case of TM polarized light, the traditional leakage mode resonance effect with narrow band and high diffraction efficiency will occur, while in the case of TE polarized light, due to the superposition of several close leakage mode resonance peaks, the reflection spectra with broad band and high diffraction efficiency will be formed. The diffraction efficiency of a one-dimensional TiO₂ SWG is calculated by the rigorous coupled-wave theory (RCWA), and the effects of grating period, height and duty cycle on the reflectivity of the grating are studied. When period, height, and duty cycle of the grating are 0.49 μm, 0.25 μm and 0.34 respectively, the SWG exhibites TE polarization selectivity. The reflectivity at 0.52 μm is close to 1, and the high reflection band (reflectivity is higher than 99.9%) had a width of 26 nm. By optimizing the structural parameters, the manufacturing tolerances of the grating period, duty cycle, and height are 1.6%, 8.3%, and 2.0% respectively, so the SWG can be theoretically used as the reflector of a vertical cavity surface emitting laser.