When manual or semi-automatic round baler working, the tractor driver have to frequently manual the machine according to the bale process at the same time of driving. The driver easily feel fatigue in this operating mode for a long time, so the consistency of the bale’s density can not be guaranteed. And there may be wrong operation. In this article, we use the model 9YG-1.2 large round baler as a research prototype. We study the information collection and processing of the baler’s key parameters. For example, the roller speed、bale density and the state of pick-up、dropfloor position、net wrap system、bale pusher etc. Based on the above, we study the method and theory of automatic operation control for round baler. The main content include the relationship between the pressure of the bale chamber closed cylinder and the density of the bales, the logical relationship between automatic net wrap system、unloading bales and closing tailgate, the necessary conditions for controlling the action of each part. In the end, the entire baling process is fully automated operated by the control system. In addition, when the machine opening and closing the tailgate, the switch storage time is longer and the efficiency is low. Therefore, we add a differential cylinder to improve the opening speed of the tailgate. The tailgate is operated through an independent hydraulic system. In order to simplify the hydraulic system, we integrate all the hydraulic systems of the bale chamber control system、dropfloor system and pick-up system into only one hydraulic system, and this hydraulic system is controlled by a terminal computer. The experiments of 9YG-1.2 large round baler with the control system show that the driver only needs to manipulate the start and stop of the tractor according to the alarm，and can continuously bale the hey in equal density without any other operation. The control method not only reduces the strength of the driver, the work efficiency has also been increased at least 1.5 times.