A novel cross-linked poly(methyl methacrylate) (PMMA)-based copolymer was first synthesized by bulk copolymerization of methyl methacrylate (MMA) and 1-oxo-2,6,7-trioxa-1-phosphabicyclo[2.2.2]octane-4-acrylate (PEPAA) at low temperature (80 degrees C), which is a facile and green method. The structure of PEPAA was confirmed by FTIR, H-1 NMR, and P-31 NMR The morphology and structure of copolymers were characterized using FTIR and SEM. Due to the chain transfer during copolymerization, a cross-linked network is successfully introduced into the copolymers and the gel fraction increases as the PEPAA content increases, as evinced by the results from Soxhlet extraction and DMA. The copolymers exhibit relatively high transparency and significant improvements to the mechanical properties, thermal properties, and fire retardancy when compared to PMMA. From the mechanism analysis, the cross-linked network plays a key role in the improvements to the mechanical properties and thermal properties. For enhanced fire retardancy, char formation during degradation caused by PEPAA is the main factor.