Disclosed is a polyester yarn that can be used in a fabric for an airbag. In particular, a polyester yarn having a diethylene glycol content of 1.1 to 2.5 wt % and initial modulus of 100 g/d or less, a production method thereof, and a fabric for an airbag produced therefrom are disclosed. The polyester yarn has excellent moisture and heat resistance and light resistance, and maintained excellent mechanical properties after long-term aging under high temperature and high humidity conditions. Therefore, when applied to a fabric for an airbag, the polyester yarn provides excellent packing property, shape stability, and gas barrier effect, and the impact applied to a passenger is minimized, thereby safely protecting the passenger at the same time.