A novel phosphorus-containing siloxane compound, 2, 2-dimethyl-1, 3-propanediol-(3'-triethoxysilanepropyl) phosphoramidate (DPTP), was synthesized and applied to prepare flame-retardant cotton fabrics with sol-gel method. The structure of DPTP was characterized by FTIR, H-1 NMR, and P-31 NMR. After coating with DPTP, the limiting oxygen index value of cotton fabrics increased to 30.1%. Besides, the total heat release decreased and the char-forming capability was significantly improved, which were measured by TG and cone calorimetry test. In the vertical flammability test, the DPTP-coated cotton fabrics exhibited no afterflame and afterglow with less than 15 cm of char length. The volatile pyrolysis products were also detected by TG-IR, and the results showed DPTP-coated cotton fabrics owned both condensation and gas phase flame-retardant mechanism.