As a bio-based polyamide, polypentamethylene adipate (PA56) is restricted in many fields due to its flammability. In this research, flame-retardant PA56 copolymers (FRPA56s) were designed and successfully synthesized with the introduction of 9,10-dihydro-10-[2,3-di(hydroxy carbonyl)propyl]-10-phosphaphenanthrene-10-oxide (DDP) unit. The thermal properties, thermal degradation and flame behavior of the resultant copolymer were investigated by DSC, TGA, LOI, UL-94 and cone calorimeter tests, respectively. Compared to PA56, the peak value of heat release rate and total heat release of FRPA56 containing 1.2 mol% DDP decrease by 35% and 19%, respectively, indicating the high flame-retardant efficiency of DDP. It is also found that FRPA56 with only 1.0 mol% DDP passes V-0 grade in the UL-94 test while its thermal performance is almost the same as homopolymer PA56. The pyrolysis products in both the condensed and gas phases were further systematically analyzed by SEM, Raman, FTIR, X-ray photoelectron spectroscopy and TGA-GC/MS. The flame-retardant mechanism was also proposed for FRPA56s.