The invention discloses an online 
monitoring system for performances of transformers based on 
wide area synchronous measurement, and the 
system comprises a plurality of CT mutual inductors, a CVT 
transformer, an EVT 
transformer, an ECT 
transformer, a PT transformer, a CVT parameter acquisition module, an environmental parameter monitoring module, a merging unit, a 
clock synchronization module, anonline monitoring device, an 
electric energy collecting terminal, and an acquisition main 
station, wherein the 
current transformer CT, the capacitive 
voltage transformer CVT and the electromagnetic 
voltage transformer PT achieve the analog independent 
butt joint with the online monitoring device. The electronic 
voltage transformer EVT, the 
electronic current transformer ECT, the CVT parameter acquisition module and the environmental parameter monitoring module are connected with the merging unit, and the merging unit and the online monitoring device achieve the digital independent 
butt joint.The 
clock synchronization module is respectively in 
butt joint with the merging unit and the online monitoring device, and the output end of the online monitoring device is connected with the acquisition main 
station through the 
electric energy collecting terminal. When the 
system is in use, the 
system can synchronously monitor the insulating performance, error characteristics and environment parameters of the plurality of transformers.