The invention provides a tilling depth detection device on a rotary tiller in the field of agricultural machines. The tilling depth detection device comprises an angular displacement sensor, a fixed plate and a moving plate. The fixed plate is fixed to a rack of the rotary tiller, a shell of the angular displacement sensor is fixed to the fixed plate, the moving plate is connected with a signal input shaft of the angular displacement sensor, connecting rods are fixedly connected to the moving plate, the moving plate is connected with the connecting rods through elastic elements, the elastic elements and the connecting rods are arranged on the same side relative to the angular displacement sensor, and an arc carriage plate is hinged to the bottom ends of the connecting rods. The tilling depth detection device is high in tilling depth detection precision, simple in structure, convenient to install, low in cost and capable of being applied to tilling depth detection work of agricultural machines.