Abstract: In order to study the influence of sand and dust deposition on the surface potential and electric field of the composite insulator string, based on the finite element method, the cleaning and sand covering model of the composite insulator string for 110 kV was established, and the changes of the surface potential and electric field distribution along the surface of the composite insulator were analyzed under the different conditions of sedimentary sand and dust. The results show that the composite insulator along the surface potential and electric field distribution is affected by geometric shape of the insulator surface. When the insulator covers the sand layer along the surface, the impact on the potential along the surface is very small, but the electric field along the surface will fall at the sand cover, and the closer to both sides of high and low voltage, the greater the decline, and the decline is independent of the thickness of the sand layer. The sand-free belt in the sand layer will increase the electric field strength along the surface of the insulator here, and the closer to the insulator on both sides of the high and low voltage, the greater the increase, and the increase is related to the width of the sand-free belt and the thickness of the sand layer.