We synthesized an acrylic acid 4‐[(10‐oxo‐10H‐9‐oxa‐10λ5‐phospaphenanthren‐10‐yl)‐phenylamino‐methyl]‐phenyl ester (DOPO‐AC), a novel Schiff base derivative based on 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene‐10‐oxide, and evaluated its utility as a flame retardant for polystyrene (PS). The synthesized DOPO‐AC was characterized by Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic resonance spectroscopy, and high‐resolution mass spectrometry. The reactive vinyl groups of DOPO‐AC further copolymerized with styrene to produce PS/DOPO‐AC. Thermogravimetric analysis showed that DOPO‐AC could clearly improve the char residue formation. By comparing the data of T5% and T10%, it was found that the thermal stability of the PS/DOPO‐AC copolymer was reduced. We investigated the influence of DOPO‐AC on the flame retardancy of PS by determining limiting oxygen indexes, conducting vertical burning tests (UL‐94), and carrying out thermogravimetry combined with differential thermal analysis. These results revealed that PS containing 5 wt% DOPO‐AC had a UL‐94V‐0 rating and a limiting oxygen index of 29.6%. Residues of PS/DOPO‐AC copolymer after the UL‐94V tests were analyzed by scanning electron microscopy, X‐ray photoelectron spectroscopy, and FTIR spectroscopy; and the results allowed us to propose a flame retardant mechanism for PS/DOPO‐AC copolymer. Taken together, our results prove that DOPO‐AC is an effective flame retardant for PS.