To obtain a polyacrylonitrile (PAN) material with highly thermal stability and flame retardance properties, a novel P(AN‐co‐DMVP) copolymers were successfully synthesized by randomly copolymerization. According to thermogravimetry analysis, it revealed that incorporating dimethyl vinylphosphonate (DMVP) into the PAN chains can perform outstanding thermal stability. Furthermore, the limiting oxygen index increased from 18.6% to 30.0% with addition of 22.6 wt% DMVP. The results of cone calorimetry tests certified that the peak‐heat release peak and total heat release of P(AN‐co‐DMVP22.6) were remarkably dropped by 70% and 33%, respectively, suggesting excellent flame retardance. The relationship between copolymer structures and flame‐retardant performance was systematically investigated by X‐ray photoelectron spectroscopy, Raman spectroscopy, thermogravimetric analysis‐Fourier transform infrared and Pyrolysis gas chromatography/mass spectrometry. A comprehensive flame‐retardant mechanism has been proposed that DMVP plays a role in both the gaseous phase and the condensed phase. During combustion, some of the phosphorus‐containing compound were released in the gaseous phase. The rest of phosphorus groups could create sufficient nucleophilicity to attack the nitrile groups of polymers and hence promote the cyclization reaction in the condensed phase. Both of the condensed‐phase and gaseous‐phase effects played important roles in the flame‐retardant PAN copolymers.