Based on the coupled-mode theory,the complex characteristic equation of long-period fiber gratings coated with metal film and sensing film is set up. By using the perturbation method,the complex roots are obtained. The results are coincident with the results of Reference[5]. The curves of intensity and the transmission spectrums are significantly influenced by the extinction coefficient. The proportion of the even mode′s intensity in core is much larger than that of the odd mode′s intensity. Also,the amplitude of the attenuation peak of the even mode is much larger than that of the odd mode. So the even mode is taken as the research object when the metal coated long-period fiber grating sensor is optimized. Furthermore,the influence of metal and sensitive film parameters on the resonant characteristics is analyzed in theory. The results show the metal film thickness,sensitive film thickness and refractive index influence the resonant wavelength,which shifts towards the short wavelength with the increase of the metal film thickness,sensitive film thickness and refractive index. And there is a jumping phenomenon for film parameters,so the film parameters corresponding to the jumping phenomena must be discarded in actual applications of long period fiber gratings (LPFG).