Abstract: Due to the poor compatibility between ammonium polyphosphate (APP) flame retardant and anion polyurethane, the resulting polyurethane still produces a large number of droplets when burned. Phosphorus-based flame retardants were used to modify anion waterborne polyurethane for flame retardant enhancement. The optimal dosage of the two flame retardants, APP and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) derivatives (DOPO-PHB), and an economically efficient flame-retardant modification system were investigated. The flame retardancy and thermal degradation performance were examined through limiting oxygen index (LOI), vertical burning test, and thermogravimetric analysis. The mechanical properties of the resulting film were determined using a tensile testing machine. Results indicate that a LOI value of 27.8% and a V-0 rating in the vertical burning test were achieved when 1% APP and 4% DOPO-PHB were added. Without melting drops, the mechanical properties of polyurethane were improved, and its compatibility was enhanced.