A novel hyperbranched phosphorous containing poly(amidoamine-organosilicon) (P@PAMAMOS) was successfully synthesized and the structure was characterized using Fourier transform infrared spectra (FT-IR), 1H NMR and 31P NMR. The cotton fabrics was grafted with P@PAMAMOS by sol-gel methods. The results of P@PAMAMOS modified cotton fabrics were proved by FT-IR and scanning electron microscopy (SEM). The thermal degradation of blank cotton fabrics and P@PAMAMOS modified cotton fabrics were probed by thermogravimetric (TG) analysis, the phenomenon showed that modified cotton fabrics has exquisite thermal stability, when the temperature reached 700℃，the char residues of A10 treated cotton, B10 treated cotton, C10 treated cotton were 29.5%, 34.5%, 28.8%, respectively. The combustion properties of cotton fabrics were tested by limiting oxygen index (LOI) and vertical burning test. The untreated cotton fabrics was completely burned and the LOI value reached 18.0%, however the char diameter of 20wt% P@PAMAMOS modified cotton fabrics was reduced 3.2 cm and LOI value increase to 31.2%, this means that P@PAMAMOS modification shows desirable flame retardancy.