In this paper, we studied the synthesis, characterization, and flammability of the water-soluble, nonhalogenated organophosphorus flame retardant (FR) dimethyl-[1,3,5(3,5-triacryloylhexahydro)triazinyl]-3-oxoPropylphosphonate for application to cotton fabrics by nonpolymeric network The FR was synthesized in a one-step reaction from dimethyl phosphite and 1,3,5-triacryloylhexahydro-1,3,5-triazine under mild conditions, and its structure was fully characterized by H-1, C-13, and P-31 nuclear magnetic resonance (NMR), mass spectrometry (MS), and Fourier transform infrared spectroscopy (FT-IR). The finishing conditions to achieve a covalent bond between the FR and fibers were optimized, and the FR capability of the treated cotton was investigated by limiting oxygen index (LOI), flame test, thermogravimetric analysis (TGA), and micro cone calorimeter (MCC) tests. This study showed that fabrics treated with the FR are effectively flame resistant, and the FR property is enhanced by covalent bonding of the FR to fibers.