Thermal effect of ultraviolet laser in fabricating and spectrum of fiber Bragg grating (FBG) was experimentally researched by phase mask method. Results indicated that, getting the same photosensitive fiber had accumulative exposures by the way of phase mask straight exposure, the grating reflectivity will not be linearly increased with the exposure level, but will be diminished, which is called “bleaching”. The process of bleaching had some connection with the interval of multiple-exposure. The shorter of interval was, the longer time of bleaching process will take. On the condition of having the same quantity of illumination, the higher exposure frequency rate of laser was, and the less time the exposures take, the grating reflectivity will be higher. During scanning exposal to make uniform FBG, on the condition of having the same exposure parameter, differ of scanning directions leads to different regularities of distribution of side lobes. Thermal-induced chirping in FBG fabrication was proposed for the first time.