In this paper, The flame retardant 2-hydroxyethyl-2-methyl-3-(6-oxido-6H-dibenzo[c,e][1,2]oxaph-osphinin-6-yl)propanoate(DOPOHM) was prepared by 9-Oxa-10-phospha-phenanthr-ene 10-oxide(DOPO)，and the flame retardant composites was prepared using acrylate resin(AR) as matrix and DOPOHM as the flame retardant. The thermal stability and flame retardancy of composites was characterized by TGA and Limit Oxygen Index(LOI), and flame retardancy mechanism was studied by SEM, XPS and EDS. The decomposition activation energy(Ea) of composites was calculated by Horowitz-Metzger theory. The result show that Ea of DOPOHM/AR composites improved as DOPOHM content increased. The DOPOHM exertinhibited effect on thermal degradation of polymer chain, and catalyze matrix formed dense char layer covering the surface of composites to be barrier to prevent heat and decomposition products escaped. The LOI of materials is found to be 26 and class V-0 of UL-94 test is passed when DOPOHM content is 25 wt%.