According to the tests and researches on model sample, this paper analyzes the effects of flashover voltage when the silicone rubber surface has one or more water drops and observes the flashover process of the clean silicone rubber surface covered with condensing water in a controlled fog chamber with a high-speed video camera. The results show that water can only exist as discrete drops on the silicone rubber surface because of its hydrophobicity, and the field is intensified at the interface between the water drop, air and silicone rubber, it will produce intense partial discharges and intermittent partial arcs. Then water drop will be elongated by electrical field force because of part loss of hydrophobicity around the drop. Consequently the field will be intensified further and it cause the reason unknown flashover.