In response to the flammability issue associated with waterborne polyurethane (WPU), this paper presented the design of a phosphorus-containing hyperbranched silicon resin ( PHPSi), which has Si—O—Si backbone. PHPSi was then used to modify WPU to create PHPSi / WPU. Infrared spectroscopy, nuclear magnetic resonance, and other techniques were used to analyze the structure of PHPSi. Performance tests and analyses were conducted on the modified WPU, including those measuring mechanical strength, thermal stability, limiting oxygen index (LOI), and cone calorimetry. The results show that PHPSi is successfully synthesized, and the WPU modified with 10% PHPSi exhibits excellent comprehensive properties. The tensile strength reaches 75 MPa. Compared with pure WPU, the limiting oxygen index ( LOI) increases from 21.8% to 25.7% . The vertical burning test indicates that the modified WPU could self-extinguish after ignition and that the content of molten droplets is significantly reduced. The total heat release and smoke production are reduced by 36.4 MJ/m² and 1.1 m², respectively, and the residual mass at 600 ℃ is as high as 18.9% .