Disclosed is a semiconductor laser spectral beam combining device for for realizing spectral width compression through two-time diffraction by using a grating and a reflecting element. The device comprises a semiconductor laser light source, a slow axis collimating cylindrical lens, a transforming lens, a grating, a reflecting element and an output coupling mirror. In the invention, the single grating is used as a diffraction element, the reflecting element reflects the beam which is diffracted by the grating for the first time, the beam is secondarily dispersed when passing through the grating again, and the dispersive power is doubled. Through the feedback of the output coupling mirror, the laser of each unit of a semiconductor laser array is locked at a different wavelength to obtain a laser output of high power and high brightness. The device of the invention overcomes the limitation of the gain line width of a gain material in the traditional spectral beam combination to the number of beam combining units, can increase the number of beam combining units within the gain line width and the high-efficiency diffraction wavelength range of the grating, increases the output power significantly, reduces the resonant cavity length with a certain spectral width, makes the laser structure compact and improves laser mode stability.