A simple route to synthesize a new type of charring agent in high yield is reported. The polyphosphate oligomer polyphosphate poly(6-oxido-6H-dibenzo[c,e][1,2]oxa-phosphinin-6-yl phenyl phenylphosphate) (POBPP) was synthesized by addition reaction between 10-(2,5-dihydroxyl phenyl)-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO-BQ) and phenyl dichlorophosphate. The structure of this product was characterized by means of the Fourier transform infrared spectra (FTIR) and nuclear magnetic resonance spectroscopy (H-1-, C-13- and P-31-NMR. The molecular weight was determined by Gel Permeation Chromatography (GPC). By blending this flame retardant resin with diglycidyl ether of bisphenol-A (DGEBA) resin, a flame retardant and anti-dripping epoxy resin was obtained. Thermal stability of flame retardant (FR) and cured composites was studied by the Thermal Gravimetric Analysis (TGA). Flame retarding performances of this epoxy were characterized by limiting oxygen index (LOI) and vertical burning test (UL94). Morphology of residue was investigated by a scanning electron microscope (SEM). The results reveal that FR improve the thermal stability of epoxy resin and help matrix to leave more residue by forming net structure in char. Graphical abstract This new flame retardant shows vapor phase mechanism at lower loading but condense phase mechanism at higher loading by resulting a large amount of char. This property helps epoxy resin to pass fire test, especially in UL94 test.