The resonant levels can be introduced into GeTe by In element, however, the effect of its microstructure on thermoelectric properties still remained unclear. In this study, a series of Ge_1-xIn_xTe samples were prepared by smelting-quenching-annealing combined with spark plasma sintering (SPS). The XRD, SEM, laser thermal conductivity instrument and thermoelectric performance analysis system (ZEM-3) were applied to study the microstructure and thermoelectric properties. Results show that, with the incorporation of In content, the unit cell volume decreases, and Herringbone structure has become smaller and grain boundaries increase, which result in a decrease in the lattice thermal conductivity. Thereby, a minimum thermal conductivity of 2.16 W·m ^-1·K ^-1 is obtained. Meanwhile, In doping introduces the resonant levels and decreases the carrier concentration, so the Seebeck coefficient and the power factor increase. Consequently, the maximum ZT value of 1.15 is obtained in the 0.03 sample at 600 K, which is 26.4% higher than that of GeTe. This indicates that the thermoelectric properties of Ge_1-xIn_xTe can be effectively improved by the microstructure regulation.