A polyester resin composition for injection molding, a method for manufacturing the same, and an injection molded product using the same are provided. An inherent viscosity (IV) of the polyester resin composition is between 0.5 dL/g and 0.85 dL/g. The polyester resin composition, based on a total weight thereof being 100 wt %, includes: 50 wt % to 80 wt % of virgin polyester chips and 20 wt % to 50 wt % of modified recycled polyester chips. The modified recycled polyester chips include 65 wt % to 72 wt % of terephthalic acid, 28 wt % to 31 wt % of ethylene glycol, and 0.1 wt % to 5 wt % of isophthalic acid.