A new phosphorus-containing dicarbamates, di(N-carbomethoxylaminomethyl)benzyl phosphine oxide was first synthesized from tetrakis(hydroxymethyl)phosphonium sulfate (THPS) via five step reactions (Scheme 1), and its structure was proved by FTIR, ( 1 H, 13 C, and 31 P) NMR, and MS. A series of thermally stable phosphorus-containing polyureas were synthesized via trans-ureation polycondensation of the new monomer with aliphatic diamines, including poly(propylene glycol) bis(2-aminopropyl ether), respectively. All of the above polyureas were fully characterized by inherent viscosity, GPC, solubility tests, and thermogravimetric analysis. The thermal gravimetric analysis results showed that the initial breakdown temperature of all phosphorus-containing polyureas was observed at above 265°C while complete weight loss occurred in the region of 406-465°C, which shows all polyureas have good thermal stability. GPC results showed that the maximum number-average molecular weight of the synthesized polyureas reached 59,100 g mol. The solubility tests results demonstrated that incorporating the ether moieties into the polymer main chain can enhance the solubility of polyureas in organic solvents.