Lead-free tin telluride (SnTe) has been viewed as one promising solid thermoelectric material for recovering waste heat in recent years. In this work, SnTe alloys doped with excessive In and Sb have been synthesized by melting, quenching and spark plasma sintering. The Seebeck coefficient has been enhanced by synergistic effect based on resonant levels and increased carrier effective mass especially at low and middle temperature range, and then, the power factor is enlarged. With the reduced electrical and lattice thermal conductivity via co-doping, the total thermal conductivity is decreased. Intrinsic point defect and more grain boundaries lead to reduction in the lattice thermal conductivity through the co-doping. In addition, as the doping level is near the solubility limit, the 200-600nm, In-rich precipitations have been detected in Sn0.848Sb0.14In0.012Te alloy, which can further reduce the lattice thermal conductivity. Thus, the lowest lattice thermal conductivity of 0.96 Wm(-1)K(-1) is obtained at 800K. Finally, the maximum figure of merit zT of similar to 0.8 at 800K has been obtained for Sn0.848Sb0.14In0.012Te alloy, and a relative high average zT of similar to 0.45 in 300-800K is achieved due to the zT improvement in the low and middle temperature range which indicated that SnTe is a promising candidate for the thermoelectric application.