Abstract: The present invention comprises a partition-composition method and its equipment for online detection of transient stability of interconnected power system. The method consists of three parts. Firstly, according to the dynamic response data of the generators at the actual time, the key parameters of transient stability analysis of local areas can be obtained from local dispatch control center based on the wide area measurement system. Secondly, the key parameters of entire grid can be obtained using the uploaded parameters from the local area. Lastly, using the partition-composition method, parameters of an adjoint power system of the entire grid can be obtained. The equipment consists of 4 modules, including the first obtained module, transport module, the second obtained module and the composition module. Using these modules, transient stability data of entire grid can be obtained exactly without the limitation of network topology, system model and parameters.