Based on the coupled-mode theory, the mode transition in long-period fiber grating (LPFG) coated with double-layer overlays including metal film and refractive response characteristics of overlay thickness have been studied. According to the excitation condition of surface plasmon resonance (SPR) and the occurrence condition of the mode transition, it is pointed out that its SPR and the mode transition do not occur at the same time, based on which, the changes of the efficient index of LPFG coated with double-layer overlays including metal film with sensing film thickness are analyzed, and it is obtained the mode transition zone of LPFG coated with double-layer overlays including metal film is wider than that of coated LPFG, and the slope of the efficient index of the first transition in the mode transition zone is bigger than that of coated LPFG, which means that there are higher responses for the variations of the sensing film thickness in this zone. The influences of metal film thickness on the refractive index of LPFG coated with double-layer overlays including metal film are further investigated. The result shows that the slope of efficient index increases gradually with the increase of the metal film thickness in the first transition of the first mode transition zone. The response characteristics of transmission spectrum of LPFG coated with double-layer overlays including metal film with sensing film refractive index in the first mode transition are investigated. The results indicate that sensitivity of sensing film refractive index of LPFG coated with double-layer overlays including metal film is higher than that of coated LPFG, and the resolution of the sensitivity of sensing film refractive index can be 10-6.