The invention particularly discloses unsaturated polyester resin for polyester concrete. The unsaturated polyester resin has a formula comprising the following components: phthalic anhydride, maleic anhydride, recoverable-grade PET flakes, 1,2 propanediol, ethylene glycol, polyhydric alcohol, benzoic acid, styrene, a polymerization inhibitor, an antioxidant, paraffin, an alcoholysis aid, copper naphthenate and p-benzenediol. The invention further discloses a preparation method of the unsaturated polyester resin which is prepared through an alcoholysis reaction, a condensation reaction and a dilution process. Compared with the prior art, the unsaturated polyester resin disclosed by the invention is low in viscosity, high in reaction activity, short in post-curing time, resistant to acid, alkali and water, and excellent in mechanical property, further, the recoverable-grade PET flakes are adopted, therefore, the cost is greatly reduced, and the production period is shortened.