Abstract: In order to get efficient flame-retardant agent, a series of phosphonyl propionamide was synthesized. The fragmentation pathway of 3--(O-methyl-O-alkylphosphonyl)propionamide was studied by electrospray ionization mass spectrometry (ESI-MS/MS). Some different fragment pathways between M+H⁺ and M+Na⁺ were observed in the ESI-MS/MS. The characteristic fragmentation pathways of protonated title compound are losing a molecule of NH₃ and C₃H₆, further losing a molecule of CO; The character of sodium adduct of title compound are losing a C₃H₆, then losing a CH₃OH. The mechanism of the possible fragmentation is advised.