In this work, a novel phosphorus–nitrogen flame retardant (CN‐DOPO), which was successfully prepared via a facile method of graphitic carbon nitride (g‐C3N4) and 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene‐10‐oxide (DOPO). Polyamide 6 (PA6) composites were then prepared through melt blending with different ratios of g‐C3N4/DOPO and different contents of CN‐DOPO. CN‐DOPO was characterized and confirmed using X‐ray diffraction (XRD), Fourier transform infrared spectra (FTIR), and scanning electron microscopy (SEM). PA6/CN‐DOPO composites were investigated by limiting oxygen index (LOI), vertical combustion (UL‐94), thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), and thermogravimetric analysis‐infrared spectrometry (TGA‐IR). The results indicated LOI value was increased from 23.4% to 29.2% and successfully passed UL‐94 V‐0 rating when the presence of CN‐DOPO increased to 15 wt%. Moreover, mechanical properties of PA6/CN‐DOPO composites had not deteriorated significantly due to CN‐DOPO having better dispersion and compatibility in PA6. The morphological of char layer revealed that CN‐DOPO formed a highly integrated char layer surface during the combustion process, as compared to that of pure PA6. Meanwhile, CN‐DOPO decomposed to release gases could inhibit decomposition by diluting flammable volatiles. Accordingly, the special gas‐condensed phase synergistic flame‐retardant mechanism of PA6/CN‐DOPO was proposed. This work indicated that CN‐DOPO could endow PA6 with highly effective flame‐retardant and good mechanical performance.