The invention relates to a high-reliability CO2 gas sensor. The CO2 gas sensor consists of insulating ceramic, electrodes, sensitive layers and a heating unit; the insulating ceramic is of a shape of a cuboid which is hollow in the length direction; the electrodes are two ring-shaped copper electrodes, are respectively positioned at both sides of the insulating ceramic in the length direction, and cover the front lateral surface, the rear lateral surface, the upper surface and the lower surface of the insulating ceramic; the widths of the electrodes are 0.5cm; the heating unit is positioned inside the hollow structure of the insulating ceramic; the sensitive layers comprise sensitive layers A and sensitive layers B; porous structures formed by a pore-forming agent calcium carbonate are arranged in the sensitive layers A and the sensitive layers B; one sensitive layer A and one sensitive layer B are sequentially arranged on the upper surface of the insulating ceramic from inside to outside; one sensitive layer A and one sensitive layer B are sequentially arranged on the lower surface of the insulating ceramic from outside to inside; sensitive materials in the sensitive layers A are HoFeO3 nano powder; sensitive materials in the sensitive layers B are SnO2 powder.