The inharmonic lattice vibration induced by the lone pair electrons of In⁺ in the InTe compound produces its intrinsically low thermal conductivity, thus InTe compound shows a great potential serving as an intermediate temperature thermoelectric material. However, its poor electrical transport properties result in an inferior thermoelectric performance. In this study, a series of single-phase In_1+xTe (x = 0, 0.001, 0.003, 0.005, 0.01) polycrystalline samples were prepared by a melting-annealing process combined with spark plasma sintering. The influence of In content on the electronic and thermal transport properties for In_1+xTe compounds was systematically studied. As the temperature rises, the predominant carrier scattering mechanism changes from grain boundary scattering to acoustic phonon scattering, leading to an unusual semiconductor-to-metal transition in In_1+xTe samples. Positron annihilation spectroscopy and electrical transport properties demonstrate that In vacancies are the main source for the charge carrier. Adding extra In effectively suppresses the concentration of In vacancies, reduces the carrier concentration and improves the Seebeck coefficient of In_1+xTe samples. The power factor of the In excess samples in the test temperature range is greatly improved in comparison with that of the pristine InTe sample. In_1.005Te sample achieves a maximum power factor of 0.60 mW·m^–1·K^–2 at 585 K, which is approximately 40% higher than the pristine InTe sample. In addition, the In excess sample maintains a thermal conductivity as intrinsically low as the thermal conductivity of pristine InTe, and the total thermal conductivity of the In_1.01Te sample at 773 K is 0.46 W·m^–1·K^–1. Owing to the improvement of the power factor and the low thermal conductivity, the ZT value of the In excess sample is greatly improved in the entire measure temperature range. A maximum ZT value of 0.71 is attained at 750 K for In_1.003Te sample, and a maximum ZT_ave of 0.39 is achieved for In_1.005Te sample in a temperature range of 300–750 K, which is about 23% higher than that of pristine InTe sample.