The invention discloses an impregnant used by a capacitor, and relates to the technical field of capacitor manufacturing. The impregnant used by a capacitor is prepared by the following ingredients: 53 to 57 parts of soybean oil, 43 to 47 parts of olive oil, 8 to 10 parts of vitamin E, 50 to 52 parts of accessory powder, 35 to 37 parts of bentonite, 20 to 22 parts of clay, 15 to 17 parts of plant ash, 5 to 7 parts of polyisobutylene, 12 to 14 parts of hydrogen peroxide, 15 to 17 parts of acetic acid, and 25 to 27 parts of concentrated sulfuric acid. The impregnant used by the capacitor and the preparation method thereof have the beneficial effects that the preparation is convenient and simple, the ingredients are easy to obtain, the equipment has little investment, operation is easy, and the prepared impregnant used by the capacitor is environmental friendly, free of pollution, good in use effects, strong in high temperature-resistance, good in heat radiation, safe, reliable, and durable in use.