The invention relates to a high-temperature-resistant copolymer oil well cement retarder and a preparation method thereof. The preparation method comprises: respectively weighing four polymerization monomers such as 2-acrylamide-2-methylpropanesulfonic acid, diallyldimethylammonium chloride, maleic anhydride or itaconic acid or fumaric acid, and acrylic acid or methacrylic acid according to the mass ratio of 45-80:5-17:7-18:8-20, dissolving the first three monomers in water, then adding sodium polyphosphate and diphenylcarbazide, then successively adding a chain transfer reagent and an initiator, and finally dropwise adding the forth monomer, performing heating reaction and adjusting the pH value, so as to obtain the copolymer retarder. The invention also provides the high-temperature-resistant copolymer oil well cement retarder prepared by the above preparation method. The retarder is resistant to a high temperature up to 230 DEG C and is weak in dispersibility at a high temperature, the cement paste strength develops fast at a low temperature, the addition amount of the retarder has good linear relation with the cement paste hardening time, and the cement paste comprehensive performance is good.