With the implementation of China’s western development strategy, the number of tunnels located in high geothermal regions has significantly increased. Thus, the cooling of high geothermal tunnels has become a key issue. This paper investigates the cooling effect of sprayed polyurethane foam (SPF) with different properties and under environmental conditions. A theoretical model is proposed to predict the cooling effect of SPF. Accurate characterization of the cooling effect along with the polymer properties and temperature based on the relationship between thermal conductivity and temperature can help to establish relationships such as the thermal insulation-temperature relationship. The results reveal that the experimental and theoretical estimation of the cooling effect of SPF exhibits an error of less than 10 %. Therefore, a theoretical model is developed on the thermal-insulation performance of SPF. The temperature inside the thermal insulation layer (TIL) decreases with increasing SPF thickness and convective heat transfer coefficient (CHTC). There is an optimum foam thickness range (5 ∼ 10 cm) for the most stable cooling effect. In addition, the long-term stable thermal insulation performance proves that the polymer can be considered as a thermal insulation material, as the temperature inside the TIL remains at 30 °C for 30 days. Furthermore, a TIL thickness calculation formula and a TIL thickness sizing table can be completed to guide on-site construction.