In this paper, by combining polyurethane waste and solid waste coal gangue, the proportion of factors such as polyurethane cuttings, spontaneous combustion coal gangue powder, polycarboxylate superplasticizer and resin rubber powder was determined, and the mixture ratio was designed according to three levels. Adding suitable additives to the mixture is beneficial to enhance its strength and durability, and also helps to save cement consumption and improve working performance. Through the tests of compressive strength, tensile strength, thermal conductivity, dry density and softening coefficient, the influence order of admixture on thermal insulation and mechanical properties of materials was determined. The optimum content of four influencing factors in orthogonal experiment was determined, which was 35% of polyurethane pellets, 6%–10% of resin rubber powder, 40% of spontaneous combustion coal gangue powder and 0.5% of polycarboxylate superplasticizer. The main properties of the prepared insulation slurry are dry density: 190–220 kg/m3, thermal conductivity: 0.035–0.042 W/m·K, compressive strength: 0.277 MPa, softening coefficient: 0.51, tensile strength: 0.176 MPa. The results show that the prepared thermal insulation slurry has good thermal insulation performance, and also meets the mechanical performance requirements of thermal insulation slurry.