The invention brings forward an active splitting strategy space reduction method and system based on trend tracking. The method comprises the following steps: at an online monitoring phase, according to acquired real-time operation data of a power system, calculating electric energy supplied by each generator to the load of each node, and analyzing ownership of non-generator nodes under a current trend; and at a real-time monitoring phase, dividing all the generators into I coherent generator groups, according to the ownership of the non-generator nodes under the current trend, analyzing ownership of any one non-generator node under a current coherent condition, and determining the non-generator nodes belonging to islands where the corresponding coherent generator groups are disposed. The method provided by the invention can effectively reduce the search space of a splitting strategy, greatly improves the search speed of an active splitting strategy, reserves sufficient time for stable control of the power system, and accordingly guarantees the stability of the power system and the power supply continuity of the loads.