In this study, we use the femtosecond laser line-by-line method and a 50 × dry objective to study the influence of laser pulse energy on grating fringes. The pulse energy of 150 nJ, the inscribe speed of 100 μm/s, the line length of 50 μm, and the period pitch of 1.994 μm are selected as writing parameters. A mid-infrared FBG with narrow bandwidth and high reflectivity is prepared based on a fluoride fiber. The central wavelength is 2964.34 nm and the reflectivity is up to 99.27%. The mid-infrared FBG with a central wavelength of 3090 nm and a reflectivity of 99.12% is prepared by adjusting the period pitch of this FBG. And it is expected to achieve a long band FBG writing by this method. This work is beneficial to the construction of an " all-fiber" mid-infrared fiber laser, which is of great significance to promote the autonomy of the core device of a mid-infrared fiber laser in China.