A novel preparation of aluminum diethylphosphinate (AlPi) as a novel class of phosphorous based flame retardants was carried out with gas-liquid free-radical addition reaction under atmospheric pressure. The ethylene was bubbled into the reaction mixture with the addition of the initiator. With this novel method the gas-liquid contact surface and the initiator concentration in the gas-liquid interface were increased largely, and the yield of the final product was improved to at least 80%. The molecular structure of the samples was confirmed by FTIR, 1H NMR and 31P NMR spectroscopic analysis. Thermal stability of the final products was investigated in detail by TG-DTA.