In order to solve the problems of high damage and low cutting rate of carrots after the top cutting in the process of carrots combined harvesting, a single disc to top cutting device was designed and its main structure and working principle was described. On the base of statistical analysis of carrot basic physical properties, the main structural parameters of the cable-stayed guide flushing to top mechanism were designed, such as the distance between the two flushing base plates, the length of flushing section and the angle of the oblique edge of the flushing base plate, etc., with which, the purpose of accurate flushing the carrot stem leaf was achieved. Taking the curve of tibia of mantis forelimb as the original form to be fit and optimized and applied to the cutting edge of the movable cuter and straight-fixed cuter of the disc to construct the mechanical model of mechanism-carrot stem leaf, through which to verify and determine the condition for efficient cutting. By kinematic analysis to set up the kinematic model of the disc cuter in the direction of moving ahead, through which, the feeding speed of the carrot and the rotating speed of the disc cuter were determined as the factors affecting the device operating performances. Taking the carrot feeding speed and the disc cuter rotating speed as the testing factor, the damage rate and cutting rate of carrot stem leaf as the testing index, the two factors and five levels quadratic orthogonal rotation combination test was carried out. The testing result was analyzed and the optimal combination of operating parameters were determined that, when the rotating speed of the conveying belt wheel was 113r/min, the rotating speed of the cuter was 156r/min, the top cutting device functioned the best with the damage rate and the cutting rate of carrot up to 0.53% and 95.41% respectively, and the flushing degree up to 94.10%, whose field verification testing was met well. The device was operated stably.