Locating and splitting the disturbance source is the most efficient way to deal with forced power oscillation. In the initial stage, comparing with the oscillating measurement data of the bus that is close to the disturbance source, the one that is further away is phase lag because of the inertia units. The oscillation starting characteristic is considered to identify the disturbance source from suspected subareas. The scheme is validated by simulations. By combining the characteristic of disturbance source of forced power oscillation islanding control, two separation control schemes are designed. One is splitting the subarea which disturbance source is located in based on actual subarea division of the power systems. The other is determining the splitting surfaces according to the proposed adjustment rules which are applicable to disturbance source islanding control.