According to coupled mode theory, the reflected polarization dependent loss (RPDL) property of fiber Bragg grating (FBG) under lateral force has been studied deeply. The numerical simulation of the RPDL under different lateral force is developed, and the simulation results show that the response of RPDL is very sensitive to lateral force. Peak height and peak position change greatly with force. Conducting a large number of experiments, experimental results accord with theoretical analysis; moreover, the results show the changing curves of RPDL peak height and peak position within a certain range are in good linear relationship. Therefore RPDL can achieve the sensitive monitoring to the changes of force field information, and a new method for FBG to demodulate exactly is provided.