The invention relates to regenerated polyester fibers and a preparation method thereof. The preparation method comprises the following steps: firstly, enabling reclaimed polyester and a polyester oligomer melt to react to obtain a primary depolymerized melt; then enabling the primary depolymerized melt and ethylene glycol to react to obtain a deep depolymerized melt; then carrying out TVOC (Total Volatile Organic Compound) removal and cracking functional group repairing on the deep depolymerized melt in a vertical type reactor to obtain a low-TVOC-content polyester oligomer melt; then carrying out pre-condensation polymerization on the polyester oligomer melt to obtain a low-TVOC-content pre-condensation polyester melt to obtain a low-TVOC-content regenerated polyester melt; finally, enabling the regenerated polyester melt to enter a melt direct spinning system to prepare the regenerated polyester fibers. The method provided by the invention has moderate whole flow length and technology control difficulty and has high efficiency; in the finally prepared regenerated polyester fibers provided by the invention, the content of TVOC is 30ppm to 300ppm and the content of nonylphenol and isomers thereof is 80ppm to 200ppm.