In this paper, a novel nonlinear double-exposure technique for fabricating fiber Bragg gratings with triangular spectrum is proposed. The spectrum of fiber Bragg grating can be controlled by double exposure technique. The first exposure is used to control the resonant wavelength of fiber grating by directly irradiating the naked photosensitive fiber. The second one is to form the grating to control its reflectivity through an ordinary Bragg phase mask. Considering the multi-decaying-exponential relationship between photo-induced index increment of photosensitive fiber and its irradiating dose, nonlinear double-exposure profiles are calculated by our self-designed software. The triangular fiber Bragg grating is obtained with DCS-01 photosensitive fiber by using a scanning system. As a result, the bottom width and the reflectivity of the grating is 1.24 nm and 0～64%,respectively. As a demodulation device, a triangular fiber Bragg gratings is very important in a fiber grating sensing system.