In this paper broad sense grating equation and broad sense chromatic dispersion formula that can be applied to reflection and transmission regions of phase grating in arbitrary oblique angle of incidence are discussed based on the vector diffraction theory. At the same time, the theoretical and numerical analyses for the chromatic dispersion capabilities of the phase grating are made to confirm the reciprocal restricted relation among the incident angle, the incident azimu thal angle, the incident wavelength, the grating period, the refractive index of the substrate in the diffraction area, and the order of diffraction,then the re stricted inequality of grating can be constructed. On the other hand, all possib ilities of improving the chromatic dispersion are presented.