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In this study, chitosan (CS) from fish waste was used to prepare bio-based environmentally friendly flame retardants. The hydroxyl group (-OH) of CS and the ammonium group (NH 4 +) of ammonium polyphosphate (APP) formed CS-APP, which was to serve as the core material. Melamine–formaldehyde (MF) resin was synthesized as the shell material through microencapsulation technology with CS-APP to prepare the novel core-shell structured flame retardant, which was subsequently introduced into polyurethane (PU) to prepare the composites. This study employed thermogravimetric analysis (TGA), the limiting oxygen index (LOI), UL-94, cone calorimetry tests (CCT), thermogravimetric analysis-FTIR (TGA-FTIR), SEM, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, smoke concentration test and tensile test to characterize the structure, thermal properties, flame retardancy, char analysis and mechanical properties of the composites. The TGA results revealed that after the addition of flame retardant, the char yield increased from 0.5 wt% (pristine PU) to 12.0 wt% (the composite), and the thermal stability of the pristine PU was improved. Furthermore, the LOI increased from 18.2 % to 28.4 %, and the UL-94 level increased from fail to V-0 for the composites. These results revealed that adding MF(CS-APP) to pristine PU resulted in high flame-retarding performance of the composites. [Display omitted] [ABSTRACT FROM AUTHOR]