Based on the mode-reorganization mechanism of long-period fiber grating induced by the increase of film thickness, the influence of cladding radius on the characteristics of long-period fiber grating -based film sensor was investigated. The results show that the reduction of fiber cladding can improve the sensitivity of film sensor in a great manner for given film parameters and enlarge the dynamic range of the response of film sensor to the variation of film thickness. Moreover, the sensitivity-increasing effect caused by cladding reduction is degraded by the increase of film thickness. In experiment, the cladding was reduced with hydrofluoric acid and the organic film of PAH/PAA was deposited by electrostatic self-assembly method. The theoretical results are confirmed by analyzing the spectral data during film deposition. The application of this film sensor in pH sensing shows the sensitivity reaches 3.93 nm/pHU when the cladding was 39(m and film thickness was 424 nm, which was two times as high as that of common fiber cladding.