To investigate the fire prevention and suppression characteristics of coal gangue slurry grouting in goafs and the enhanced regulatory mechanisms of additives, the slurry-forming performance of coal gangue slurry was tested. The effects of heating temperature, grouting thickness, and heating duration on the surface temperature distribution characteristics were analyzed. Temperature-programmed experiments were conducted to examine the influence of various additives on the spontaneous combustion propensity of coal gangue, with a comparative analysis of the inhibitory effects between ammonium polyphosphate (APP) and other additives. The results demonstrate that the prepared coal gangue slurry exhibited no segregation or sedimentation, with a plasticity index consistent with standard grouting material requirements, confirming its superior stability. The central, maximum, and minimum surface temperatures of the slurry showed polynomial functional relationships with heating temperature. Surface temperature initially increased and then decreased with grouting thickness, with 10 cm identified as the critical thickness for temperature transition. Overall, the central, maximum, and minimum surface temperatures increased progressively with rising heating temperatures. In addition, under all tested conditions, the average surface temperature remained below 80 °C for slurries with >5 cm grouting thickness, meeting fire prevention requirements. However, the CO and CO₂ concentrations increased significantly as heating temperatures rose from 100 °C to 300 °C. At grouting thicknesses of 9–12 cm, CO and CO₂ emissions occurred only at 300 °C and decreased with increasing thickness. The coal gangue slurry modified with ammonium polyphosphate (APP) additives exhibited optimal antioxidant performance, significantly suppressing CO and CO₂ emissions, which further diminished with higher additive dosages. The findings of this study provide critical insights into the fire prevention performance of coal gangue slurry grouting and the application of additives in this field.