Polymer‐contained oily sludge is inevitably produced when the oily wastewater produced from polymer flooding (OWPF) is treated by a cationic polymer clarifier, and it severely affects oilfield production. This study designed a novel degradable cationic clarifier (DCC) based on the ester bond and prepared it in light of the poly(β‐amino ester) used in the area of generic material delivery. First, the copolymer of N‐[3‐(dimethylamino) propyl] methacryamide (DMAPMA) and diallylamine (PDMAPMA‐NH) were synthesized by radical copolymerization. Then, the DCC was prepared by Michael addition between PDMAPMA‐NH and poly(ethylene glycol) diacrylate. The synthesis conditions of DCC were investigated, and a DCC with excellent flocculant performance and good degradability was successfully prepared. At 70°C, both the degradations of DCC and the water‐insoluble polymer complex formed by DCC and the polymer used for polymer flooding can be completed in 10 h, which is beneficial for the self‐disappearance of the polymer‐contained oily sludge. The flocs formed by DCC were observed at different times, and the results confirmed that the flocs can change to an oil film and that the polymer‐contained oily sludge cannot exist for a long time. This study provides inspiration for OWPF treatment and expands the new application for poly(β‐amino ester).