In this work, 2-carboxyethyl(phenyl)phosphinic acid melamine salt (CMA, phosphorus valence +1), melamine hypophosphite (MHPA, phosphorus valence +1), melamine phosphite (MPOA, phosphorus valence +3), and melamine pyrophosphate (MPyP, phosphorus valence +5) were respectively used as flame retardants for flexible polyurethane foam (FPUF), and fire performance as well as pyrolysis behaviors of FPUF were investigated systematically. CMA and MHPA were found to have better flame retardance than MPOA and MPyP for FPUF, which mainly played a role in the gas phase due to the release of many phosphorus-containing volatiles and melamine derivatives. The types of phosphorus-containing gaseous pyrolysis products of CMA were much more than that of MHPA due to the different chemical moieties surrounding the phosphorus atom, which made it have the highest flame-retardant efficiency for FPUF. Besides this, the possible thermal degradation mechanisms of CMA and MHPA were proposed.