The invention discloses a branch potential-energy function-based power grid out-of-step separation method. The method includes the following steps that: off-line simulation is performed through setting expected faults, so that the separation sections of a power grid can be determined; a separation list is generated according to the separation sections of the power grid; the real-time transmission power of each branch and bus-bar voltage phase angles at two ends of each line branch are obtained through monitoring; a branch potential-energy function is obtained according to the real-time transmission power of each branch and the bus-bar voltage phase angles at the two ends of each line branch; and the real-time separation indexes of the branches are obtained through calculation according to the branch potential-energy function and the real-time transmission power of each branch; whether the branches achieve a separation threshold value is judged online according to the real-time separation indexes of the branches; and the separation instructions of branches which achieve the separation threshold value are determined according to the separation list. According to the out-of-step separation method provided by the invention, regional separation control centers cooperate with a provincial separation control center, so that the information of separation devices can be shared; and when the power grid is out-of-step under a serious fault, the separation sections can be fast and reliably separated, and therefore, the fault can be isolated, and the safety of the power grid can be ensured.