Herein, a novel material functionalization of fiber components is proposed, which is based on optically induced film forming. The film formation of polymer around a microfiber is achieved only by injecting an ultraviolet laser through the microfiber. The evanescent field of an ultraviolet laser, propagating along the microfiber surface, provides activation energy for photopolymerization of polymer and film formation directly onto the surface of a microfiber. The thickness of the deposited film can be controlled by adjusting the irradiation time. Additionally, the potential application case demonstrates that fiber components can be endowed with a given function by this material functionalization method. Therefore, a fiber relative humidity sensor using a polymer film-functionalized microfiber Bragg grating fabricated by this functionalization technique is demonstrated. This proposed sensor has the advantages of relatively fast response, large measurement range, and high stability.