Security and stability control systems ensure the secure operation of power systems. Existing function tests of stability control systems are typically performed in offline environment using centralized open-loop testing, which cannot effectively address complex dynamic conditions and suffers from limitations in real-time performance and adaptability. To overcome these shortcomings, this paper presents a distributed online testing method based on hardware-in-the-loop, enabling online testing with non-real-time simulation tools. First, key technologies such as synchronization, information exchange, and result evaluation in a distributed system are studied. Then, an event identification algorithm suitable for dynamic testing is designed according to testing requirements. Next, a multi-round closed-loop iteration testing platform is developed to provide comprehensive testing results and verify the effectiveness of the stability control systems. Finally, a pilot application of the proposed method is conducted on the stability control system of a sending-end power grid. Simulation tests and engineering validation confirm the performance of the proposed testing method.