The invention discloses a device for preparing a long-periodic fiber bragg grating by femtosecond laser Talbot effect and a method for preparing the long-periodic fiber bragg grating by femtosecond laser Talbot effect, and belongs to the technical field of manufacturing of a fiber sensor. The device comprises a femtosecond laser light source, first and second collimating diaphragms, an achromatic half-wave plate, a polarization splitting prism, a shutter, a reflecting mirror, first and second convex lenses, a cylindrical lens, first and second precise moving platforms, a phase grating, a to-be-processed fiber, an electric controlled three-dimensional precise moving platform, a broadband light source, a spectrograph and a computer. Based on the Talbot effect, the femtosecond laser is used for irradiating the phase grating and forming stable bright and dark alternated light intensity distribution at a preset score Talbot distance place behind the grating; the femtosecond laser is linearly focused and irradiated on the to-be-processed fiber through the cylindrical lens, and the to-be-processed fiber and the femtosecond laser are mutually acted to generate non-linear absorption; the refractive index is periodically changed, and thereby the long-periodic fiber bragg grating is prepared. The device and method have the advantages of being simple in structure, high in utilization rate of the femtosecond laser, short in processing time, and low in manufacturing cost.