Volume Bragg gratings (VBG) fabricated by photo-thermo-refractive glass are widely adopted in linewidth-narrowing of laser diode with external cavities. The model was based on finite element analyses. Through the model, thermal properties of the VBG were researched under conditions of different laser diode (LD) illumining area, different laser power and different gratings′ working temperature. When the LD power was 40 W and the working temperature was 25 ℃, the increase of the maximal temperature are 8.52, 5.07 and 2.20 ℃ at the illuminated altitude of 1.48, 0.88 and 0.28 mm. The discrepancy between the temperatures of the illuminated area and the working temperature was linear to the LD power. By improving the VBG working temperature, the gap between the actual temperature and the working temperature can be reduced. According to the research, the uniformity of the VBG′s temperature can be optimized by choosing illuminated area and LD driving conditions, which will be beneficial to acquire the LD with narrow linewidth.