Moisture is a strong polar molecule, and it could change the relaxation time of dielectric and accordingly affect the state of transformer insulation. So it is of great significance to analyze the transformer’s aging and damp condition with the method of diagnosing the micro water content of the transformer accurately. By applying the dielectric response return voltage method, combining with the characteristics of the return voltage polarization spectrum by field test to discuss the relationship between the micro water content and the characteristics of the return voltage polarization spectrum further, we have come up with an conclusion that the smaller the dominant time constant is, the more water the transformer contains; the larger the initial slope is, the more water the transformer contains. At the same time, with the increase of water content in the transformer the maximum value of return voltage will also be increased. So we can diagnose the micro water content of the transformer by analyzing the characteristics of the return voltage polarization spectrum, so as to assess the aging and damp state of oil-paper insulation transformer effectively.