Dense spectral combining (DSC) based on reflection volume Bragg grating (VBG) is an important method to improve brightness of fiber-coupled diode laser modular. Accounting for laser divergence, spectral bandwidth, wavelength and angle shifts from ideal Bragg condition, a diffraction efficiency model of reflection VBG is presented. Based on the new numerical model of diffraction efficiency, design principles and technical approach for DSC of two channels by reflection volume Bragg grating are investigated. The results show if wavelength shift is less than ±0.49 nm and angle shift is less than ±16 mrad from ideal Bragg condition, the DSC combining efficiencies are over 98.7% when the spectral full width at half-maximum (FWHM) is 0.3 nm, and the divergence angle with over 90% energy is 14 mrad.