Disclosed is an apparatus for realizing semiconductor laser spectrum beam combination by utilizing double gratings. The apparatus comprises a semiconductor laser light source, a conversion lens, a first diffraction grating, a second diffraction grating and an output coupling mirror, wherein the semiconductor laser light source emits multiple bunches of parallel light; the light beams are subjected to secondary chromatic dispersion through a diffraction effect of the double gratings; and after the light beams are fed back by the output coupling mirror, the laser of each unit is locked in different wavelengths correspondingly to overlay the respective light beams so as to realize high-power and high-luminance single-beam laser output. By taking the double gratings as a diffraction element, the dispersive power is doubled; when the apparatus is applied to laser spectrum beam combination of the semiconductor laser, the spectrum expansion width can be reduced; by adding more beam combination units to a gain curve of a semiconductor laser gain material, the output power can be improved; and meanwhile, by compressing the length of a resonant cavity, the laser can be compact in structure and the stability can be improved.