The invention relates to a preparation method of a composite insulating material containing polyurethane. The composite insulating material is prepared from the following ingredients in parts by weight: 100 parts of polyurethane, 50-150 parts of nitrile-butadiene rubber, 50-150 parts of polyacrylate rubber, 20-60 parts of carbon black and 10-50 parts of polyethylene glycol. The composite insulating material containing the polyurethane, provided by the invention, has favorable insulating property, achieves a resistance value not less than 80 M ohm.km, is good in weather resistance, has less possibility of being aged and decomposed in air, and can be widely applied to the industry of electric power and the like as an insulating material.