Abstract: Epoxy resin (EP) is inherently flammable, and existing 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO)-derived flame retardants mainly rely on non-renewable petroleum-based resources. In this work, a bio-based reactive phosphorus-containing flame retardant, DOPO-ITA, was synthesized from bio-based itaconic acid (ITA) and DOPO, integrating curing and flame-retardant functions. The chemical structure of DOPO-ITA was characterized by Fourier transform infrared spectroscopy (FTIR), proton nuclear magnetic resonance (¹H NMR), phosphorus nuclear magnetic resonance (³¹P NMR), and mass spectrometry (MS). The influence of DOPO-ITA on the curing behavior, thermal stability, flame retardancy, combustion behavior, thermomechanical properties, mechanical properties, optical properties, and flame-retardant mechanism of epoxy resin was investigated. At a phosphorus content of 2.3wt%, the DOPO-ITA_2.3/EP achieved a UL-94 V-0 rating, with the limiting oxygen index reaching 38.1%. The peak heat release rate and total heat release decreased by 70.4% and 24.1%, respectively, while the char residue increased from 3.45wt% to 7.94wt%. Meanwhile, the glass transition temperature was elevated by 18.8℃, and the tensile strength, flexural strength, and flexural modulus were enhanced by 25.8%, 13.8%, and 59.0%, respectively, whereas the elongation at break decreased by only 6.0%. Furthermore, the light transmittance at 800 nm increased from 75.49% to 87.92%. Flame-retardant mechanism analysis revealed that DOPO-ITA facilitated the formation of a compact char layer and released phosphorus-containing active radicals and inert gases such as CO₂, resulting in synergistic flame retardancy in the condensed and gas phases. This study provides a green and efficient strategy for developing high-performance bio-based reactive flame-retardant epoxy materials.