This paper studies how to effectively cut off the load and achieve coordinated control of the output of master-slave BES in the process of unplanned off-grid switching. First, a step-by-step load shedding method considering the constraints of BES ratio characteristics is proposed. This method uses the BES overload characteristics to support the remaining uncut load after off-grid for a short time. This can overcome the defect of step-by-step load shedding. Then, at a specific stage in the load shedding process, the slave BES output is dynamically adjusted based on the virtual droop/inertia control principle to effectively reduce the regulation pressure of the main control unit. An unplanned off-grid smooth switching control strategy considering the BES ratio characteristic constraint and dynamic PQ control is formed. Finally, the model is built in Matlab and verified by simulation. The results show that the proposed strategy can not only ensure that the voltage is always maintained at a high level during the switching process, but also significantly reduce the voltage fluctuation. This is conducive to the smooth transition of the switching process. This work is supported by the National Key Research and Development Program of China (No. 2017YFB0903400).