Carrageenan fiber (CAF) was prepared by a wet spinning method to develop an excellent flame-retardant material. In order to investigate the flame-retardant mechanism of CAF, a series of tests were carried out. The values of limiting oxygen index (LOI), total heat release (THR), and total smoke release (TSR) of CAF reached up to 52, 2.9 MJ m⁻², and 2.5 m² m⁻², respectively, while there was no Time To Ignition (TTI) in cone calorimeter (CONE). These results indicate that CAF exhibits superior flame-retardant performance than other alginate fibers. Thermogravimetry, differential scanning calorimetry, Fourier transform infrared spectroscopy (TG-DSC-FTIR) and pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) tests were performed in order to explore the pyrolysis mechanism. The results show that CAF absorbed more heat and produced more carbon residue than calcium alginate fiber (ALF) and agar fiber (AGF) in the process of thermal degradation. In addition, in the current work, we have explored the possible flame-retardant mechanisms which can be attributed to sulfonyl free radical (SFR), carbon residue, barium sulfate, water vapor, and carbon dioxide.