Pseudo-binary layered compound IVVI-V₂VI₃ families show great promise for application in thermoelectrics. Herein, through introducing iodine in GeSb₂Te₄, several synergistic effects come into being and contribute to outstanding thermoelectric performance. The I^Te donor-like defects suppress the hole carrier concentration from 5.72 × 10²⁰ cm⁻³ to 2.80 × 10²⁰ cm⁻³. First-principles calculations reveal that iodine doping increases the band gap from 0.253 eV to 0.302 eV and contributes to valence band convergence. Seebeck coefficient value reaches up to 135.7 μV/K at 773 K, and the power factor values are entirely boosted in the whole temperature region, reaching a maximum value of 12.4 μW·cm⁻¹·K⁻² in GeSb₂Te_3.96I_0.04. Moreover, iodine doping simultaneously reduces the lattice and electronic thermal conductivity, leading to the greatly reduced total thermal conductivity from 2.89 W·m⁻¹·K⁻¹ in pristine sample to 0.89 W·m⁻¹·K⁻¹ in GeSb₂Te_3.84I_0.16 at 323 K. Finally, a maximum zT ~1.12 at 773 K and an average zT ~0.62 over 323–773 K are achieved in GeSb₂Te_3.88I_0.12. This work puts forward an effective strategy to synergistically optimize phonon and carrier transport properties of pseudo-binary compounds through halogen doping, which may be effective in other similar material systems.