In order to solve the problem of high loss of fiber Bragg gratings (FBGs) fabricated by femtosecond laser point by point, experimental research on the fabrication of femtosecond fiber Bragg gratings was completed using the plane?by?plane method. Using the basic theory of Gaussian beam propagation, the focused femtosecond laser energy distribution was contoured with equal height and width using slit shaping technology, breaking through the difficulty of inducing circular refractive index modulation within the fiber cross?section, and ultimately fabricating low loss femtosecond fiber gratings. Comparative experiments were conducted on the spectral characteristics of FBG fabricated with different slit widths. The results showed that when using a femtosecond laser beam with a spot diameter of 5.0 mm to write FBG, the insertion loss of the FBG prepared with a slit width of 1.7 mm was reduced to 0.15 dB, and the short?wavelength loss was reduced to 0.5 dB. This verified the effectiveness of the low loss femtosecond fiber grating fabrication method based on slit shaping. A method is proposed to control the filling amount of refractive index matching solution and adjust the femtosecond laser energy for the problem of dispersion of FBG reflectivity fabricated by the slit method, and the beam focusing process is optimized to successfully reduce the dispersion of FBG reflectivity. This study is of great significance in promoting the practical application of femtosecond fiber gratings in environments with high capacity and high chain loss.