The invention discloses a fully automatic leveling structure for a 3D printer, which includes a frame, a nozzle and a heated bed plate. The frame is provided with an XY platform, and the XY platform is provided with an active synchronous wheel, a passive synchronous wheel, a guide rail and an X-axis fixed plate. The two ends of the lower part of the X-axis fixed plate are provided with guide rail grooves, the guide rail grooves are erected on the guide rails, the active synchronous wheel and the passive synchronous wheel are connected with a belt, and the belt drives the X-axis fixed plate to move back and forth on the Y-axis. The beneficial effects of the present invention are : The automatic leveling system can complete the automatic leveling of the large-format 3D printer printing platform. This method completes the distance measurement through the distance sensor, measures three points, compares the data, and uses the distance between the first support column and the second support column. The measurement distance between the measurement points is used as the reference to adjust, the measurement points are few, and the speed is fast.