The invention relates to an insulating material for bus, which comprises the following components by weight: 65-85 parts of bisphenol A-type epoxy resin, 0.06-0.11 part of nanometer organic montmorillonite, 15-35 parts of a curing agent, 280-380 parts of SiO2 micro powder, 0.01-0.05 part of ZnO micro powder, and 0.05-0.15 part of BaTiO3 micro powder. The insulating material of the invention can operate under awful weather conditions, is widely applicable to power plants, power stations, transformer substations, photovoltaic power stations, wind power generation transformer substations, is an important accessory for power transmission equipment, is an important material for processing high voltage electric power matching insulating parts, and has good insulating properties and water resistance. The invention also provides a preparation method of the insulating material.