Phosphorus-containing trimethoxysilane (DGTH) was blended with Star polyurethane acrylate (SPUA) in different ratios to obtain a series of UV curable intumescent flame retardant resins. The fire properties were characterized by limiting oxygen index (LOI) and cone calorimetry. A distinct synergistic effect was found between SPUA and DGTH. The thermal degradation was characterized by thermogravimetric analysis and real time Fourier-transform infrared spectroscopy. The TG results have been found to correlate well with the LOI results. A degradation mechanism has been suggested that the 9,10-dihydro-oxa-10-phosphaphenantrene-10-oxide (130130) group in DGTH first degraded to form poly(phosphoric acid)s, which further catalyzed the degradation of the material to form char with emission of nitrogen volatiles from SPUA, leading to the formation of expanding char. The morphologic structures of crusts of the formed chars were observed by scanning electron microscopy, confirming the synergistic effect between SPUA and DGTH.