The application provides a dynamic union resection control device and system of an automatic throw-in device of a backup power supply. The device comprises a receiving unit for receiving first real-time operating data sent by a line at the same level, wherein the first real-time operating data are used for expressing an overloading level of a line-in power supply at the same level; a calculation unit for calculating a load amount needing to be resected according to the first real-time operating data; a selection unit for selecting a load according to the calculated load amount needing to be resected and obtained through calculation; and a resection unit for resecting the load selected by the selection unit. Therefore, when the load is definitely resected by the resection unit, the load amount needing to be resected is calculated by the calculation unit according to the first real-time operating data sent by the line at the same level and received by the receiving unit, and the load is finally selected and resected according to the load amount needing to be resected and obtained through calculation, so that the load needing to be resected can be dynamically accurately calculated according to an actual demand, and the device and the system are high in intelligence.