The invention discloses a method and a system for online monitoring of dielectric loss and capacitance of a capacitor voltage transformer. According to the method, a voltage/current monitoring device is used for obtaining secondary voltage of a busbar PT, secondary voltage of the capacitor voltage transformer, piezoelectric capacitance current in the capacitor voltage transformer and primary current of an intermediate transformer of the voltage transformer and sending the obtained data to a data processing device for calculation. The method and the system have the advantages that automatic online monitoring of the dielectric loss and the capacitance of the capacitor voltage transformer is achieved, definite physical significance and accurate measurement results are obtained, and fault positions of the capacitor voltage transformer can be judged and positioned accurately; assembly and disassembly of a high-voltage primary lead wire of the capacitor voltage transformer are avoided, onsite measurement workload is reduced, and all test operations can be conducted without powering off, so that normal power supply of an electrical power system is unaffected.