The invention belongs to the technical field of engines, and relates to an automatic air door carburetor of a gasoline engine. The automatic air door carburetor comprises a carburetor body, a thermoswitch is subjected to heat control of a gasoline engine body, and a rotation shaft rotating through heating is connected with a drive arm. The drive arm drives a rocker on a main shaft controlling theaperture of a main air door of the carburetor body to rotate through a push rod so that the aperture of the main air door can be controlled. A control rod of an air pump controlled by the negative pressure in a gasoline engine air inlet pipe to act drives the rocker on the main shaft controlling the aperture of the main air door of the carburetor body to rotate so that the aperture of the main airdoor can be controlled. The rocker is provided with an arc hole concentric with the main shaft. Before cold engine start and during main air door closing, the drive end on the outer side of the pushrod is located at the front end of the arc hole. The automatic air door carburetor is arranged on the gasoline engine, the main air door can be automatically opened or closed according to the regulated time during cold engine start or heat engine start, manual intervention of the aperture of the air door is not needed, the starting action is simple, the starting success rate is high, and one-key start can be achieved.