The relationship between phase, microstructure, and transport properties can be guided by phase diagram engineering, which can investigate the high-performance thermoelectric (TE) alloys. While pristine GeTe possesses high thermal conductivity , Ge1-xBixTe alloys decreased ~ 85% to 1.2 Wm-1K-1 at 323 K. The strongly lattice distortion and high-density stacking faults were revealed by synchrotron XRD and TEM analysis, which were the main reasons for the huge reduction of . Besides the reduction of , Bi as substitution also declined the carrier concentration from 9.5 ×10^20 to 2×10^20 (cm-3) simultaneously. In summary, doping Bi instead of alloying with Bi2Te3 or Bi4Te3 in GeTe induced rhombohedral distortion, leading to low . Therefore, Ge0.9Bi0.1Te boost peak zT from 0.7 to 1.9 at 713 K.