The effect and mechanism of wheat bran cellulose (WBC) on the gelling characteristics of soy protein isolate (SPI) were evaluated. It was found that the water holding capacity, gel strength, and viscoelasticity of SPI gel were improved with the increase of WBC concentration. The addition of WBC (0.5-2.0%, w/v) stabilized the moisture phase and induced the construction of the regular and homogenous three-dimensional gel network. The Raman spectroscopy revealed that WBC addition caused a significant reduction in α-helix percentage (28.92-63.08%) (p < 0.05) with a concomitant increase in β-sheet (16.92-34.37%) (p < 0.05) and β-turn (8.09-13.54%) (p > 0.05) percentages of the pure SPI gel. Additionally, hydrogen-bonding interaction between SPI and WBC and the enhanced thermal stability were proposed in the composite gels. Overall, WBC is effective in improving the gel properties of SPI, suggesting its potential application as novel gel modifier in the food industry.