Abstract: With the finite difference method and the coupled mode theory, it was analyzed that photonic crystal fiber (PCF)-s structure parameters affected the adjusting action of long period grating (LPG)-s grating period on the resonant wavelength. The results showed that, for the same kind of PCF, it is feasible to decrease or increase resonant wavelength by increasing or decreasing grating period. If duty cycle f of PCF is increased or decreased, the resonant wavelength can be kept unchanged by decreasing or increasing grating period. If the scale coefficient M of PCF is increased or decreased, the resonant wavelength can be kept unchanged by increasing or decreasing grating period. For the PCF series only with the layer numbers of air hole increased, in the long-wavelength region, the grating period should be increased several ms to keep the resonant wavelength unchanged, while it was just the opposite in the short-wavelength region. The air holes in the inner layers greatly affect the adjusting action of grating period on the resonant wavelength, while the layers outside of the fifth layer affect the action very faintly. Using these regularities, it is portable to choose appropriate grating period, and high-effectively prepare LPG in PCF with appropriate resonant wavelength.