The closed slewing system has already been widely used in a large excavator. However the rule between its working performance and the control requirements is in need of further research. The principle of a closed slewing system of a large excavator was analyzed, and simulation model for the system was established. In order to achieve better control effect, the open-loop control and the closed-loop control were researched respectively, meanwhile the results were compared. The system pressure was oscillated around the cutting pressure during the startup process of the system, and completely overflowed without energy recycled during the braking process when the structure of open-loop control was adopted. When the structure of close-loop control adopted, the system pressure was stabilized nearby the controlled pressure during both the startup and braking processes and the excess energy was able to be recycled. The research results show that the effect of the closed-loop control structure in large inertia closed slewing system is even better. The analysis conclusions provide reference for the system design of a super large excavator.