Line fault trip criterion has been widely used in power system security and stability control equipment. The equipments have also been widely used in all levels of power grid. Whether the criterion is right or not, it relies on the relay protection device’s tripping signal. In recent years, as the relay protection device acts faster and faster, the power system security and stability control equipment sometimes can’t identify the kind of fault correctly. Based on an example of phase-to-phase fault which occurred in site, we propose an improved method of the fault-trip criterion which can be used in security and stability control system, the use of FWK-300 Stability Control Device validates the correctness, reliability and effectiveness of the new method by RTDS test. Results of the tests show that the improved method is correct,reliable and effective.