The tool, which is used in borehole trajectory control devices in highly deviated wells , gives us an effective method for drilling to the multibranched diversion in sandy well. When testing the downhole tools, the damage and failure of cantilever bearing have been found. In order to analyze the failure mechanism of the cantilever bearing, and based on theory of pipe string mechanics, the mechanical model of the spindle was established, then the mechanical behavior of the cantilever bearing under the working condition was analyzed. The failure reason of the cantilever bearing was obtained by the finite element simulation analysis. As the research shows, the deformation of basic shaft makes abrasion on the needle bearing that the left of it is worse than the right. The deformation of basic shaft cause radialinternal clearance of needle bearing and pre-tighten load, which make the bearing resist unequalSforce that generates unbalanceloading. Under the action of the friction torque, the holder has a torsion deformation, which leads to the failure of the cantilever bearing. Combined with the analysis results, the model of the cantilever bearing is presented, which can effectively improve the life of the bearing.