A cetain value, a method to analyze bending sensing rules of long-period fiber grating (LPFG) according to bending curvature, grating length and grating period was reported, based on coupled-mode theory and perturbation approximation. When the bending curvature reaches a cetain value, the transmission spectra of the bended LPFG split. The two resonant wavelengths of the splitting peaks change conically with the bending curvature and inversely with grating period, but are indepentent on grating length. The LPFG written on the B-Ge co-doping single-mode fiber proves the above conclusion.