In order to study the melting characteristics of polymethylmethacrylate(PMMA) by CO2 laser irradiation, according to the heat conduction and energy conservation principle, the theoretical model of PMMA melting was established by low-power CO2 laser radiation. The laser parameters threshold of PMMA melting was calculated and the relationship between sample temperature change and laser parameters was analyzed. By the use of digital microscopy systems, SEM and XRD instruments, the melt morphology and tissue features of PMMA sample were discussed. The results show that laser power density 31.8 to 47.7 W/cm2 could make PMMA melt and does not occur thermal decomposition, and the experimental results is consistent with the theoretical calculations of laser melting threshold, which provides reference for CO2 laser melting mechanism and molding technology of PMMA.