The invention discloses a high-arc resistance epoxy insulating material and a preparation method thereof and an insulator. The epoxy insulating material is prepared from the following raw materials in parts by mass: 1 part of composite epoxy resin, 0.51-0.55 part of curing agent methyl tetrahydrophthalic anhydride and 3.25-3.45 parts of aluminum oxide, wherein the composite epoxy resin is a mixture of phenolic aldehyde modified bisphenol A epoxy resin, polyethylene glycol modified bisphenol A epoxy resin and diphenolic acid modified bisphenol A epoxy resin. The epoxy insulating material is equivalent to traditional materials in glass-transition temperature, tensile strength and electric strength, the arc resistance reaches 237s, and after a 252kV circuit breaker is switched on/off under voltage for over 3243 times, flashover appears behind; and the epoxy insulating material has excellent arc resistance, the operation times of a high-voltage circuit breaker under voltage is remarkably increased, the service lives of internal epoxy insulating components are prolonged and reliability of the internal epoxy insulating components is improved, and the maintenance and overhauling cost of a power station is reduced.