Nine types of PMMA composite flame retardant materials were prepared by the melt blending method, and small-scale horizontal fire spread experiments were conducted on them. The effects of flame retardants of expandable graphite (EG) and carbon nanotube (CNTs) on the combustion characteristics of materials were compared and analyzed. The change rules of fire propagation characteristics parameters, such as fire propagation velocity, flame morphology, solid phase temperature and mass loss rate, were mainly studied. The results showed that EG mainly exhibited flame-retardant effect. With the increase of EG content, the fire propagation velocity, mass loss rate and surface heat flux of the composite material all decreased. CNTs exhibited a compound effect of antagonistic and synergistic flame retardancy. When the content of EG was low, the addition of 1% CNTs accelerated the fire spreading rate. Subsequently, with the increase of EG content, its antagonistic effect gradually disappeared, and finally showed the synergistic flame-retardant effect. The high thermal conductivity of CNTs and the "wick effect" promote combustion, and EG/CNTs system prevented combustion.There was a competitive relationship between them. After adding EG and CNTs, the fire spreading process showed obviously different combustion behavior. Before adding flame retardant, PMMA produced molten drips and accumulate into a pool fire, showing obvious burning characteristics of thermoplastic materials; adding EG and CNTs, a carbon layer formed, showing obvious burning characteristics of carbonizable materials.