Water absorbing action in outdoor polymer insulating materials, especially used in high humidity or heavy-rain regions, has a large influence on their surface hydrophobicity which contributes to their superior contaminant performance. Therefore, it is significant to investigate the relation between water absorbing or drying action and their surface hydrophobicity. By solving the diffusion equation and applying the Young's equation to it theoretically, an equation which can explain the relation between cosθ (θ:contact angle) and water absorbing or drying time is obtained. It is proven that the proposed equation agrees well with the experimental results of most materials except silicone rubber, which has a unique hydrophobicity recovering system. Therefore, we note the water absorbing and drying actions of silicone rubber