The invention relates to novel phosphorus-containing polyester compound composition and a preparation method thereof, and belongs to the technical field of novel compounds. The phosphorus-containing polyester compound composition is prepared from a (A) phosphorus-containing polyfunctional aromatic hydroxyl compound, a (B) bifunctional aromatic acyl chloride compound and a (C) monofunctional aromatic phenolic compound serving as an end capping agent through a condensation polymerization reaction under certain conditions, and is used as a curing agent for epoxy resin. A halogen-free flame-retarding epoxy resin cured product which is environment-friendly, low in dielectric property and dielectric loss factor and excellent in heat resistance can be obtained by reacting the novel phosphorus-containing polyester compound with epoxy groups of epoxy resin, thereby being capable of serving as a packaging material of integrated circuit plates and semiconductors.