The invention relates to a rectangular bale baler bale compactness control system and control method, and belongs to the technical field of balers. The control system mainly comprises a main controller, a terminal display, a first auxiliary controller, a pin sensor, a second auxiliary controller, an oil pressure sensor and a hydraulic executing device, the terminal display transmits bale compactness parameters inputted by a user to the main controller, the main controller reads data of the first auxiliary controller and the second auxiliary controller through a CAN (controller area network) bus, the hydraulic executing device is controlled through comparison operation, and the bale compactness is adjusted to met requirements of the user. According to the system, the bale compactness can beautomatically adjusted within the allowance range according to selection of the user, corresponding alarm reminding can be achieved according to values of the pin sensor, so that long-time overload of the baler is prevented, and the bale compactness can be displayed on the display in real time. The control system lays a foundation for intelligent development of the baler in a communication mode of the CAN bus.