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 test (LOI), cone calorimeter test (CC) and vertical burning test (UL-94). The results showed that the flame retardant effect of 5%DMMP-EG/PMMA was the best when 10 % compound flame retardant was added and the ratio of DMMP to EG was 1∶1. The sample passed UL-94 V-0 rating, and its LOI value increased from 18 % to 27.4 % compared with PMMA. The peak heat release rate (pHRR) was significantly reduced by 87 %, and the total heat release rate (THR) was reduced by 33%. The dynamic mechanical properties test showed that the glass transition temperature and storage modulus of 5%DMMP-EG/PMMA were much higher than those of 10%DMMP/PMMA, and the composite expandable graphite could 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.