The dielectric properties of cross linked polyethylene (XLPE) and liquid silicone rubber (LSR) are affected by damp. In order to explore the effect of moisture on the dielectric properties of XLPE and LSR and its mechanism, in this paper, the XLPE and LSR samples are taken as the research object, and the dielectric spectrum, breakdown field strength, moisture content and infrared spectrum of the sample after different water immersion time at 65 ℃ are measured. It is found that after immersion in water, moisture has an effect on the dielectric properties of XLPE and LSR, and moisture has a greater influence on the dielectric properties of LSR. The loss factor at 0.1 Hz is more obvious in reflecting the moisture of XLPE and LSR. The relationship model between 0.1 Hz and 50 Hz tanδ is obtained by Pearson correlation analysis. After being immersed in water, XLPE samples are mainly affected by combined water, and LSR samples are mainly affected by free water.