Abstract: As a transmissive and passive fiber-optic device, the long-period fiber grating is interested extensively due to its applications in fiber telecommunication and sensing technology. The transmission spectrum was analyzed to acquire the satisfactory fiber grating. The coupling constants between the fundamental mode and the first-order cladding modes were determined by a mode-coupling equation. With given parameters of the fiber grating, the transmission spectrum of the long-period fiber grating was obtained by simulation with Matlab. As a result, the position of the resonant wavelength of the long-period fiber grating is decided by the grating′s period. The attenuation changes with the length and refractive index of the grating.