In order to realize the automation and high efficiency labeling of round bales, a robot labeling system of round bales has been constructed. A image recognition method of round bale end face under complex background has been studied and a combined recognition algorithm is proposed. Firstly, the radius range of round bale is determined, and the watershed algorithm is used to segment the adhesion image. In order to prevent the leakage and error identification of round bale end face caused by over-segmentation, true roundness threshold and angle threshold are proposed to recognize the over-segmented images further precisely. Then the pixel coordinate centers of round bale end faces are obtained using the ellipse fitting method, and the conversion relationship between the pixel coordinates and world coordinates is completed by the calibration based on the Interpolation of Delaunay Triangles. Finally, the labeling experimental system is set up. Experimental results show that the speed of this labeling system is 20 roots per minute,and the accuracy rate is 99.8%. The system can meet the actual production needs of enterprises, and it can provide certain reference for technology development and practical application of round bale end face automatic labeling at home and abroad.