Abstract: On one type of cantilever rig, the cantilever can't be precisely locked in any position as expected, and can't meet the production requirement in some conditions, this research is based on changing this situation. In this article, the structure of the cantilever locking system and the principle of the control system are analyzed in details, it is found that there are also some hidden troubles in the design of the system. Then through the research of pertinence, the author puts forward the optimization design scheme and recommendations for improvement of the system and analyzes it. The results show that the improved locking system meets the production requirements, the cantilever can be locked in any position accurately, and the hidden trouble of the cantilever abnormal movement has been solved at same time.