Cooling load prediction is an important task in tunnel construction. Traditional tunnel cooling load prediction methods cannot meet the requirements of high-altitude railway tunnel construction environments. To provide an accurate method for predicting the cooling load of high-altitude railway tunnels, this paper established a heat-moisture coupling porous medium model, which considers the influence of surrounding rock porosity and low-pressure environment on the heat flux density of tunnel surrounding rocks. A prediction model for heat flux density in the surrounding rock of high-altitude railway tunnels was established. Analyzed the impact of the presence of seepage water and lining on the surrounding rock heat flux density, and compared the impact of different environmental parameters on the surrounding rock heat flux density. The results show that the tunnel's cooling load depends on the surrounding rock's heat flux density. The prediction model can accurately calculate the heat flow density of the tunnel surrounding rock, with a root mean square error (RMSE) of 0.573 W/m2. Surrounding rock seepage water and lining significantly impact the tunnel's heat and humid environment. Pressure has the smallest effect on the surrounding rock heat flux density. When the temperature of the surrounding rock increases by 10 °C, the initial heat flow density of the surrounding rock increases by 58.74 W/m2. In addition, whenever the tunnel target temperature increases by 4 °C or the porosity increases by 0.1, the initial surrounding rock heat flux density decreases by 23.5 and 0.14 W/m2, respectively. This research can provide new theoretical methods and reference bases for cooling load prediction in actual high-altitude railway tunnel projects.