Reactive flame-retardant dimethly allylphosphonate (DA) was prepared using dimethyl phosphite and 3-bromopropy and was grafted onto cotton fabric via ultraviolet (UV) irradiation using N,N'-Methylenebis (2-propenamide) (MBA) as a crosslinker and benzoin dimethyl ether as an initiator. The effects of DA and MBA mass concentrations on the flame retardancy of the cotton fabric were studied. The results indicated that the flame retardancy of the cotton fabric was proportionate to the mass concentrations of DA and MBA. The limiting oxygen index (LOI) of the cotton fabric could reach 27.2%, and the afterburning time and smoldering time were both 0 s. After 20 washing turns, the LOI could still reach 24.5%. The final cotton fabric exhibited better thermal stability, and the flame-retardant finishing demonstrated no significant effect on the tensile property of the cotton fabric.