Splitting can be driven by either faults（fault-driven splitting performed at the second defense line）， or trajectories（out-of-step splitting performed at the third defense line）. Under absolutely deterministic conditions， only fault-driven splitting is needed to reduce impact on power systems. However， there are many of non-deterministic factors which weaken the necessity of fault-driven splitting but emphasize out-of-step splitting and the importance of coordinating them. Multiplying the control cost of all isolated sub-systems with the splitting probability is newly defined as the risk cost of splitting control proposed as an index in this paper， and the coordination is also based on this index. Then“on-line pre-decision and real-time match” is used to improve the adaptability of fault-driven splitting. The paper also proposes an adaptive out-of-step system that consists of a control center， local splitting stations and splitting devices. The control center would detect the location of an oscillation center automatically， and coordinate fault-driven splitting and out-of-step splitting according to the splitting risk. Local splitting stations would be responsible for on-line selecting the cutting sets of the power grid. Simulation studies undertaken on power systems show that this optimal and coordinative splitting scheme is effective and practical.