To address the challenge of preventing spontaneous combustion of residual coal in goafs of thick coal seams, a long-acting composite retardant solution with MgCl₂ -Vc-SDS as the core was developed. Experiments show that the optimized formulation (MgCl₂ 7.5%, Vc 2.5%, SDS 0.025%) significantly increases the critical temperature for coal spontaneous combustion. The production temperatures of CO and C₂H₄ are delayed by 10℃ and 20℃, respectively, with an average retardation rate of 53%. The contact angle of the retardant solution is reduced by 35%, and its reverse capillary rise height is 1.5 times that of pure water, indicating significantly enhanced wetting performance. Long-term performance tests over 20 days showed a fluctuation in the retardation rate of 3.6% to 5.4%. Thermogravimetric and infrared analysis confirmed that the retardant solution functions through dual mechanisms of physical oxygen isolation and chemical modification, increasing the critical temperature of coal samples by 22℃ and the dry cracking temperature by 35.9℃. It reduces oxygen-containing functional groups by 18% and increases hydroxyl groups by 18%, establishing a dual retardation mechanism of “physical oxygen isolation-chemical inhibition”.