The recovery voltage measurement method can effectively evaluate the state of transformer oil-paper insulation, but the measurement time is too long. Thus, this paper presents a method to diagnose the insulation state by the voltage response characteristic of the polarization established at one polarization time and released in different depolarization periods, which eliminates the need for multiple voltage applications, and saves the time of measurement. The characteristics of response voltage in different depolarization periods of transformer oil-paper insulation under different moisture conditions were experimentally studied. The results show that the initial slope of the voltage response which obtained by the first discharge 0.1s after charging 2000s increases with the increase of the moisture content of the insulation, which can be used as a feature to evaluate the moisture condition of the insulation. At the same time, the DC conductivity of the medium was measured by the frequency domain analyzer and compared with the polarization conductivity calculated from the initial slope of the first discharge to further prove the validity of the feature. This measurement method provides a new research idea for the use of response voltage measurement to quickly diagnose the moisture status of transformer oil-paper insulation.