Abstract: Based on the three-layer fiber model, we found that the relation between the structural parameters and the transmission spectra of LPFG is regular variation, and the variation rules of the transmission spectra were presented. This paper only considers the couple between lowest-order cladding modes and the fundamental core mode. When radii and refractive index of fiber core and cladding increase, the resonance wavelengths shift towards shorter wavelength; When grating period increase, the resonance wavelengths shift towards longer wavelength; When grating length increase, the resonance wavelengths have no shift, but its depth decreases. Comprehensive analysis shows that the shift in the resonance wavelengths is clear, but the change of depth doesnt have distinct rules.