With the widespread use of capacitor voltage transformer(CVT) in power system, there exist a few problems, among which internal capacitance component breakdown failure is one of common problems.Now a large amount of CVT on-line monitoring system need to add primary hardware equipment,which not only would increase investment,but also might affect system security.This article use CVT secondary voltage in EMS(energy management system)that has been in steady operation in Guangdong power grid corporation, and research on the relation between CVT self-state and secondary voltage, then determine threshold of fault diagnosis through combination of theoretic calculation and statistical analysis, in the end has established CVT on-line monitoring system without addition of primary system hardware equipment,this system could prevent fault deterioration and threaten safety of system, so as to achieve the goal of increasing power system stability.