Abstract: A novel phosphorus-containing flame-retardant, intermediate 4-hydroxy(6-oxido-6H-dibenzo c, e1, 2oxaphosphinin-6-yl methyl)benzaldehyde(DOPO-HO-BD), was successfully synthesized by the reaction of 1, 4-phthalaldehyde with 9, 10-dihydro-9-oxa-10-phosphaphenan- threne-10-oxide(DOPO) in dioxane solution. The experimental results indicate that the yield of DOPO-HO-BD is 89.3% under 7 h of reaction at 30 ℃, while mol ratio of DOPO to 1, 4-phthaladehyde 1.1:1. The structure of DOPO-HO-BD was characterized with Fourier transform infrared spectrometry(FT-IR), nuclear magnetic resonance spectrometry(NMR) and mass spectrometry(MS). Thermogravimetric(TG) analysis results show that the initial decomposition temperature at 1% mass loss of DOPO-HO-BD is 211 ℃, and the amount of residue(char yield) at 800 ℃ is 29.7%