The invention provides a preparation method of polyester resin, which comprises anti-oxidant and monomer. The invention modifies epoxy resin (PEP), processes reaction between epoxy group and unsaturated polyester terminal group to form a segmented copolymer, and adds monomer with light activity to prepare photosensitive polyester resin. The modified photosensitive unsaturated polyester resin has significantly improved alkali tolerance and surface rigidity, and reduced volume shrinkage, while the surface rigidity, thermal stability and adhesion of paint film are improved.