In this paper, the physical principle of phase grating’s resonance is discussed and the basic (approximate) formulae for the resonance is presented. According to the formulae, the properties of resonance peak, including the condition for multi peak and single peak, its relation with grating film thickness, incidence angle, and grating period, is also discussed. The results are consistent with those calculated with rigorous theory. An efficient design method for controlling the single peak resonance wavelength is found. In this paper, the physical principle of phase grating’s resonance is discussed and the basic (approximate) formulae for the resonance is presented. According to the formulae, the properties of resonance peak, including the condition for multipeak and single peak, its relation with grating film thickness, incidence angle, and grating period, is also discussed. The efficient are consistent with those calculated with rigorous theory. An efficient design method for controlling the single peak resonance wavelength is found.