The resonant wavelength of long-period fiber gratings (LPFGs) is very sensitive to the ambient refractive index. LPFGs will have many potential applications on biochemistry sensors and environment monitor system. At present, LPFGs chemical sensors can only measure the medium, which has lower refractive index than that of the fiber cladding, however, the detecting range can be greatly enlarged if the LPFGs coated with Langmuir-Blodgett thin film are used. LPFGs will have more extensive applications with the mature of the L-B thin film technology. In this paper, the spectrum performance of the resonant wavelength of LPFGs varying with the changes of the ambient refractive index (1< n < 1.8) is theoretically analyzed. As the ambient index is increased, each resonance wavelength first shifts toward the shorter-wavelength direction and then disappears where the value of n is about 1.45. When the ambient index is larger than that of the cladding (~1.45), the resonance wavelengths reappear at slightly longer wavelength than those measured in ambient air. According to the mode coupling method, the theoretical four-layered fiber model is developed on the relationships among the resonant wavelengths of LPFGs coated with L-B thin film, the refractive index and thickness of the L-B thin film, and ambient refractive index. The shift of the resonant wavelength is calculated through numeric method and is presented graphically.