The invention relates to an auxiliary mechanism of a peanut combine harvester, in particular to an automatic depth limiter of the peanut combine harvester. The automatic depth limiter is characterized in that a ground-contour-following device (2) is arranged closely to the ground, an ultrasonic transducer (1) is disposed above the ground-contour-following device (2), an ultrasonic ranging module (3) of the ultrasonic transducer (1) is arranged aiming for a measurement datum plate (12), signals acquired by the ultrasonic ranging module (3) are transmitted and back fed to a controller (4), control signals of the controller are transmitted to a magnetic valve (5), and digging depth of a digging shovel is adjusted through a hydraulic tubing and a double-acting hydraulic cylinder (6); a guide tube component, formed by a lower guide tube (17) and an upper guide tube (15) with opposed openings, is vertically fixed onto an axle of a land wheel (9), a pressure spring (10) is arranged in the guide tubes, and a welded plate (14) and the horizontal measurement datum plate (12) are fixed onto the top of the upper guide tube (15). Through design of the ground-contour-following device, ultrasonic waves are prevented from being transmitted directly to the ground, ranging accuracy is improved, anti-interference performance is high, depth limiting accuracy is high, and breakage rate of peanut harvest is greatly reduced.