Subwavelength high contrast grating (SHCG) is considered as an optical resonator for understanding its resonant filtering mechanism clearly. The transmission spectrum of SHCG is investigated by employing temporal coupled mode theory. The calculated result is in good agreement with the simulation obtained by using the rigorous coupled-wave analysis method. In addition，the influence and mechanism of the filter′s transmittance on incident angle，grating width and temperature are also investigated, respectively. The simulated results show that the spectrum linewidth of SHCG filter is mainly controlled by the incident angle. However, the spectrum linewidth is insensitive to the minor change of the grating width and the operating temperature.