The invention discloses an oilwell cement retarder, which is applied to the technical field of oil drilling, well completion or well cementation. A preparation method for the oilwell cement retarder comprises the following steps of: adding 20 to 50 parts of polyamine compound, 10 to 30 parts of phosphorus trichloride or phosphorous acid and soft water into a reaction vessel, heating to a temperature of between 50 and 90 DEG C, activating for 1 to 3 hours, dripping 10 to 30 parts of aqueous solution of formaldehyde into the vessel, after finishing the dripping of the aqueous solution of formaldehyde, raising the temperature to between 100 and 120 DEG C, and reacting for 1 to 4 hours; and adding 10 to 40 parts of aminosulfonic acid compound into the vessel, after complete dissolution, adding 10 to 30 parts of epoxy compound, and keeping the temperature to continue the reaction for 2 to 4 hours so as to prepare the oilwell cement retarder. The retarder can adjust the cement paste thickening time in a temperature range of between 50 and 180 DEG C, is suitable for low, medium and high density cement paste systems, has good salt tolerance, is suitable for fresh water, seawater, saline and the like, and can be widely applied to complicated well conditions of shallow shafts, deep wells, ultra deep wells, horizontal wells, extended-reach wells and the like.