The transmission mechanism of hollow-core photonic bandgap fiber gratings is studied based on finite element method and coupled local-mode theory. The model of holes collapse is built, the distributions of effective refractive indexes and mode coupling coefficients are calculated in the structural change region, and the transmission spectrum is obtained by the coupling between fiber core fudmental mode (LP01) and cladding mode (LP11). Furthmore, the impacts of the fiber and gratings structure parameters involving in the air hole diameter, air hole spacing, grating period, the number of periods, the depth of cladding collapse and the collapse way on resonant peaks are studied. The results indicate that the resonant wavelength shifts to short wavelength direction varying with the increase of air hole diameters and the gratings period, the decrease of air hole spacing and collapse depth. The resonant wavelength shows no obvious shift with the increase of the number of periods. Otherwise, the resonant wavelength shifts towards short wavelength direction where the fiber is heated by single-side collapse comparing with symmetric collapse.