Based on the coupled-mode theory, the mode transition in absorption film coated long-period fiber grating (LPFG) and response characteristics of refractive index are studied. The response characteristics of effective index of cladding mode for coated LPFG with overlay thickness are discussed, and the mode jump and mode transition are unified in mode field distribution and the essence of waveguide transmission. Based on this result, the change in absorption film coated of real and imaginary of effective index of cladding mode for LPFG coating absorption film with different overlay thicknesses and overlay refractive indexes is analyzed. The one step and two step of mode transition of the high order and low order cladding mode are given and explained. The effect of film extinction coefficient to mode transition zone is further investigated. The results show that the film extinction coefficient has little influence on the mode transition zone. Finally, the response characteristics of refractive index in LPFG coated absorption film are investigated in the mode transition zone and near the mode transition zone. The results indicate that resolution of film refractive index in mode transition is one order larger than that in vicinity of mode transition, and the resolution can be also available to 10-5 in mode transition detected by transmittance change.