Aiming at the difficulty of fabricating large-area nanograting structures with femtosecond laser in one step, the direct writing method of femtosecond laser pulse using slit-spatial shaping is proposed in this paper. By conducting a study on the parameter dependence of nanograting structures on the single-crystal silicon surface with the processing system, the optimized conditions of incident shaping femtosecond laser—energy density of 8.00 μJ/cm², scanning speed of 9 mm/s, and slit width of 0.40 mm—are obtained. By using SEM, AFM and other microscopic characterization methods, it is indicated that the fabricated nanograting structure has an extremely high width (41.20 μm), greatly improving the fabrication efficiency of large-area nanograting structures in one step. This study provides a certain reference for the current research on efficiency optimizing and performance enhancing of femtosecond laser direct writing systems.