Aiming at the problems of large discharge and low utilization rate of red mud (RM), and limited coal rock body reinforcement, this paper studied the effects of RM as a supplementary cementitious material on properties of magnesium phosphate cement-based grouting material (MPCG) and the bonding mechanism between MPCG and coal gangue was also revealed. The results showed that the addition of RM could improve the flowability of slurry, the compressive strength and water resistance of MPCG. When 15.0 wt% RM was added, the compressive strength at 28 d of MPCG could reach 22.8 ± 0.3 MPa. However, its addition increased the drying shrinkage of matrix. The hydration heat results showed that RM could delay the hydration reaction and increase the cumulative heat release of the MPCG system. After adding RM, the density of the MPCG matrix was improved and no obvious cracks appeared. In addition, after RM was added, the flexural strength of MPCG and coal gangue bonding was improved. The bonding interface between MPCG and coal gangue was closely bonded, and no obvious cracks appeared in the matrix.