The commercialization and popularization of biodiesel are mainly restricted by the utilization in cold areas and long-term storage due to the poor low-temperature fluidity and oxidation stability. In order to synchronously improve the low-temperature fluidity and oxidation stability of biodiesel, a bifunctional polymethacrylate (PMA) type copolymer was synthesized by grafting amine antioxidant 1-phenylnaphthylamine onto PMA originally. The grafted copolymers showed excellent performance in reducing the cold filter plugging point (CFPP) and pour point (PP) of B20 (20 vol% soybean biodiesel +80 vol% diesel fuel) and the maximum reduction were 13 and 10 °C respectively at the addition of 1500 ppm, which exhibited lower values of 5–8 °C and 4–6 °C respectively than un-grafted copolymers. The induction period (IP) of B20 treated with 2000 ppm of grafted copolymers was prolonged by 17 h compared with pure B20. In addition, the attenuation of the cold flow properties of B20 samples during the oxidation process was studied, and the mechanism of bifunctional copolymer to improve the properties of B20 was explored.