Abstract: Our theoretical simulation calculation is expected to provide an approach for realizing narrow line width for a spectral beam combined diode laser bar. The conventional spectral beam combining structure was improved by adding a pair of anamorphic prisms. The anamorphic prisms reduces the output spot of the semiconductor laser line array, thereby theoretically reducing the incident angle to the grating and reducing the whole spectral line width .With the help of anamorphic prisms the whole wavelength of a spectral combined laser bar can be narrowed down to 3.2 nm from more than 10 nm. Compared with the method to reduce the spectral line width by increasing the focal length of cylindrical lens, the advantage of this approach is to ensure the overall structure of the whole spectral beam combining within 500 mm, which provides sufficient amount of feedback and guarantees the beam quality and efficiency of beam combining.