Moisture is one of the main factors affecting insulation performance of oil-paper insulated bushing and the diagnosis and analysis of the moisture condition of oil paper insulated bushing have important engineering value. In order to study the frequency domain dielectric characteristics of oil-paper insulated bushing under different moisture conditions

the frequency domain dielectric responses from 0.1 Hz to 1 k Hz of normal bushing and core moisture bushing is tested. The distribution relationship between core bushing degree and test results is obtained and two characteristic values of moisture defects

average dielectric loss deviation E（Δlg（tan δ））and real capacitance ratio KC′are extracted. The study results show that the moisture

as the strong polar substance

will lead to reduction of insulation resistance of bushing and increase of the dielectric loss. The influence of core moisture on insulation performance of bushing is more significant than that of insulating oil due to the difference in hydrophilicity between insulating paper and insulating oil. The bushing with core moisture has the feature of E（Δlg（tan δ））>0.23 and KC′>1.05% within the frequency range from 0.1 Hz to1 kHz. On this basis

such defect judgement as the dielectric loss increment less than70% and capacitance variation less than 5% within the frequency range of 0.1 Hz to 1 k Hz is formulated and the actual application effect is good. The research results provide a reference for site analysis and diagnosis of moisture de-fects of oil-paper insulated bushings.