A novel photopolymerizable poly phosphinate (poly ethyl (4-acrylamidebenzyl)phosphinate, P-NH-AC) flame retardant was synthesized by a carbene polymerization and characterized using Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance spectroscopy (NMR), and gel permeation chromatography (GPC). The effect of P-NH-AC on the kinetics of photopolymerization, thermal stability, combustion behaviors, and physical and mechanical properties of the UV-cured materials were investigated by real-time infrared spectroscopy (RT-IR), thermogravimetric analysis (TGA), thermogravimetric analysis/infrared spectrometry (TGA-IR), the limiting oxygen index (LOI), and the cone calorimetric test (CCT). For the systems with P-NH-AC, the thermal stability was improved with the increase of the P-NH-AC; however, the final residue of all systems was low. The addition of 5% P-NH-AC increased the LOI from 29.0 to 32.0. The addition of P-NH-AC significantly decreased the heat release rate (HRR), total heat release (THR), and total smoke production (TSP) of the resin. Moreover, P-NH-AC can also improve physical and mechanical properties of the materials.