A mixture (denoted as PX) of phosphates was prepared by the reaction of phosphoric acid and a kind of bicyclic pentaerythritol phosphate (PEPA, 1-oxo-1-phospha-2,6,7-trioxabicyclo[2.2.2]-4-hydroxymethyl octane). Then PX was successfully copolymerized with PEG200 to achieve a novel phosphorus-containing polymer (denoted as PX-co-PEG). 31P NMR, 1H NMR and FT-IR analysis results showed that there were 38.7 wt.% PEPA phosphates and 61.3 wt.% pentaerythritol biphosphates in the PX mixture. The result of GPC indicated that the molecular formula of PX-co-PEG could be noted as PX2.0PEG3.0. Fire protection test results proved that PX-co-PEG modified amino resin applied on the plywood boards effectively extended the protection time over flame. The decomposition process of fire resistant coating was studied by thermogravimetric analysis and FT-IR which could be divided into three characteristic temperature stages. The solid residue was kept to be 14.6 wt.% at 787 °C.Highlights► Phosphoric acid was used to react with PEPA to synthesize PEPA phosphates (PP). ► PP was copolymerized with PEG200 to achieve oligomer written as PP2.0PEG3.0. ► The degradation of PP-co-PEG could be divided into four stages. ► The degradation of PP-co-PEG modified amino resin could be divided into three stages.