A flame retardant with enhanced phosphorus-nitrogen content, 5,5-dimethyl-2-(phenyl(phenylamino)methyl)-1,3,2-dioxaphosphinane 2-oxide (DPPO), was synthesized by the reaction of 5,5-dimethyl-1,3,2-dioxaphosphinane 2-oxide (DDPO) with N-benzylideneaniline. The structure of DPPO was characterized by nuclear magnetic resonance (H-1 NMR and P-31 NMR) and Fourier transform infrared (FT-IR) spectroscopy. The thermal stability of DPPO was characterized by thermogravimetric analysis (TGA). The solubilities of DPPO were measured in different solvents including ethyl acetate, methanol, chloroform, acetonitrile, acetone, 1,2-dichloroethane, 1,4-dioxane, dichloromethane, tetrachloromethane, benzene, tetrahydrofuran, and isopropanol at temperature ranging from 278.15 to 347.15 K by the gravimetrical method. The mole fraction solubilities of DPPO in the above-mentioned organic solvents were correlated as the Apelblat equation, and the calculated values with equations shows good consistency with the experimental values. The root-mean-square deviation was less than 0.1%, and the average relative error was less than 0.04 in all of the experiments.