We used dichroic mirror dense spectral beam combing to model and analyse the impact on incident angle, centre wavelength and thin-film transmission. Further, we developed an experimental study on three sub-beam combing wavelengths of 969, 976, 981 nm, achieving an output power of 311.9 W, a beam combining of 95.88%, a brightness of 58.42 MW/(cm²·sr), a spectral width of 12 nm, and a beam quality deterioration of less than 1.06 times. Compared to traditional beam combing based on the dichroic mirror, we can decrease the spectral width from several tens of nm to only 7 nm, and the combing channels substantially increased, allowing us to achieve higher brightness and power LD output relatively easily. In addition, when compared with dense spectral beam combing of about 0.5 nm, this type of beam combing has the disadvantages of a wider spectral interval, but this method can play the role of secondary beam combing and the output from dense spectral beam combing can be regarded as sub-beam. These findings are important in terms of increasing the power and brightness of direct diode lasers.