Phosphorus-containing polymers have been found wide applications in many fields. In this work, a new kind of phosphorus-containing monomer with α-hydroxy phosphonate 4-((diethoxyphosphoryl)(hydroxy)methyl)phenyl methacrylate (PHMA) was firstly synthesized, and then copolymerization of PHMA and methyl methacrylate (MMA) was carried out via reversible addition–fragmentation chain transfer (RAFT) polymerization. The copolymerization kinetics and successful chain-extension reaction using the resultant PPHMA-co-PMMA confirmed the features of “living”/controlled radical polymerization. According to the thermal gravimetric analyzer (TGA) test, the thermal stability of the resultant phosphorus-containing copolymers increased significantly compared to the conventional methacrylate polymer materials due to the attachment of the phosphonate groups into the side chain of the copolymers. In addition, its superior flame-retardant performance was verified by the micro-scale combustion calorimetry (MCC) test. Meanwhile, the hydrophilicity of the phosphorus-containing copolymers was reflected by water contact angle measurement.