Abstract: Polymethyl methacrylate (PMMA) is a widely used polymer material. In order to change its flammability, a series of PMMA composites containing dimethyl methylphosphonate (DMMP) and expandable graphite (EG) were prepared by in-situ polymerization. The flame retardant properties of PMMA composites were studied by limiting oxygen index (LOI) test, cone calorimeter test and vertical burning test (UL-94). The results show that the flame retardant effect of 5%DMMP-EG/PMMA is the best when 10wt% compound flame retardant is added and the mass ratio of DMMP to EG is 1∶1. The sample passes UL-94 V-0 rating, and its LOI value increases from 18.1% to 27.4% compared with PMMA. The peak heat release rate (pHRR) is significantly reduced by 87%, and the total heat release rate (THR) is reduced by 33%. The dynamic mechanical properties test show that the glass transition temperature and storage modulus of 5%DMMP-EG/PMMA are much higher than those of 10%DMMP/PMMA, and the composite expandable graphite can improve the softening of the material caused by the addition of phosphate ester. Finally, the flame retardant mechanism of the composites was analyzed by TG-IR, infrared spectroscopy at different temperatures and Raman spectroscopy of carbon residue. The good flame retardant effect was due to the synergistic flame retardant effect of DMMP and EG in the gas phase and condensed phase.