Cation doping has been mainly used to enhance the performance of rocksalt GeTe thermoelectric materials. However, its counterpart, anion doping, is still in infancy. Here, we provide a theoretical insight into modifying the anion sites of GeTe for obtaining a better thermoelectric performance, envisaging its coordinate-based bonding mechanisms accounting for carrier and phonon transport characteristics. To support our point, we experimentally synthesized anion I doped GeTe samples, which show comparably optimized electrical and thermal properties compared to Sb or Bi doped GeTe samples. The (Bi, I) co-doped GeTe samples further obtain an enhanced figure-of-merit from 0.8 to 2.5 at 675 K, which can be assembled as an 8-couple thermoelectric generator to yield a conversion efficiency of ~11.6% under a temperature difference of 500 K. This work rationalizes the potential of anion doping for high-performance GeTe thermoelectrics.