Aqueous-based polyurethane (PU) with a carboxyl pendent group was prepared from a conventional PU prepolymer process. Its carboxyl group served as the ionic center for stabilizing the aqueous PU dispersion and as the curing site for post-curing reaction. A curing agent was synthesized from the substitution reaction of phosphorus oxychloride or phenylphosphonic dichloride by aziridine. Each compound consisted of two aziridinyl groups and the phosphorus composition. Its aziridinyl group reacted with a PU carboxyl group and its phosphorus content was introduced to this PU curing system simultaneously. Those compounds served as both curing agent and flame retardant for the PU curing system. The physical and mechanical properties of the resulting PU were improved corresponding to the curing dosage. The thermal properties and the flame inhibition behaviors of those post-cured PUs were evaluated by measurements using thermogravimetric analysis, dynamic mechanical thermal analysis, a limiting oxygen index, and a cone calorimeter.