Based on the wide area measurement system（WAMS）， an adaptive out-of-step detecting composite criterion and a corresponding out-of-step splitting framework are proposed. The main U cosφ criterion， which reflects the changing track of oscillation center voltage， detects the synchronism loss. While the auxiliary criterion， which reflects changes of three-phase currents， guarantees the reliability of splitting action. Furthermore， the auxiliary criterion is independent of the oscillation center. These two criterions are used together for determining out-of-step accurately and duly. Simulation results of a practical power system in China have demonstrated that the proposed scheme can adapt to changes of operating conditions， capture the position of the out-of-step center dynamically and cope with the shortcomings of a single criterion.