The invention discloses an active splitting method and system for a local power grid. The method comprises the steps that a main grid step-out cross section information substation sends rejection warning information to a coordination control master station; after receiving the rejection warning information, the coordination control master station obtains power angle information of all electric generators of a main grid power transmission cross section sending end and receiving end corresponding to a main grid step-out splitting device; the coordination control master station calculates equivalent power angles of the main grid power transmission cross section sending end and receiving end; if a difference value of the equivalent power angles of the main grid power transmission cross section sending end and receiving end is larger than a preset power angle threshold, the coordination control master station selects a local power grid power transmission cross section conforming to active splitting, and issues an active splitting command to a local power grid splitting execution station related to the local power grid power transmission cross section; after receiving the active splitting command, the local power grid splitting execution station splits the local power grid power transmission cross section. According to the method and the system, existence and stable operation of the local power grid are guaranteed, and large-area power outage caused by rejection of the main grid step-out splitting device can be defended easily.