The influence of moisture on dielectric properties of insulation pressboard at microwave band based on micro-strip ring resonant (MRR) test was studied. The micro-strip ring resonator based on NX9320 high-frequency sheet was designed, and samples of transformer insulation pressboard with different degrees of moisture were prepared simultaneously. The insertion loss curves of resonant devices were obtained by Pico vector network analyzer. Based on the analytical model, the dielectric constant and loss tangent of pressboard samples at 1~6 GHz frequency band were calculated. MRR test can obviously reflect the difference in dielectric properties of insulation pressboards with different moisture contents and only several seconds are taken. At the same time, the results were compared with traditional frequency-domain dielectric spectroscopy (FDS) test at 1mHz to 5kHz that needs nearly one hour. It is believed that MRR test has a bright prospect in the on-line moisture evaluation of insulation pressboard.