Taking a utility tunnel in Baotou City, Inner Mongolia as the research object, the optimum ventilation area corresponding to different pipe diameters in the thermal cabin of the comprehensive utility tunnel under different ventilation modes was studied by combining experiment with CFD numerical simulation. The results show that the length of ventilation area of two ventilation modes increases with the increase of pipe diameter of thermal pipeline. The optimum ventilation areas of DN300mm, DN400mm, DN500mm and DN600mm pipe diameters under natural ventilation and mechanical ventilation modes are 160 m, 166 m, 178 m and 200 m respectively, which can meet the comfort requirements of inspectors. Under the ventilation mode, the optimum ventilation areas of DN300mm, DN400mm, DN500mm and DN600mm pipe diameters are 360 m, 370 m, 385 m and 400 m respectively, which meet the requirements. The research has certain guiding significance for the actual ventilation design of the utility tunnel.