The introduction of special alloy nanoparticles and elemental elements in the matrix can synergistically optimize the thermoelectric properties of SnTe. Here, MgAgSb alloy nanoparticles and 0.5 mol % In are co-doped into SnTe to optimize electrical and thermal properties, which finally make the thermoelectric performance greatly improved. The improvement of Seebeck coefficient results from the valence band convergence because of the presence of Ag, Mg, and Sb atoms and a resonant state generated by doping In element. In addition, the carrier mobility is enhanced owing to the decrease of Sn vacancies caused by the doping. A high power factor (17.67 μW cm-1 K-2) can be obtained in Sn1.025In0.005Te-3%MgAgSb sample at 835 K. Doping multielement alloy could also effectively improve the thermal properties by scattering phonons in each frequency band, which cannot be realized merely by doping single element. This strong phonon scattering is derived from point defects and particles produced by doping MgAgSb. Extremely low thermal conductivity is achieved with the value of 1.046 W m-1 K-1 at 835 K. Finally, we acquire a high ZT of 1.41 at 835 K in Sn1.025In0.005Te-3%MgAgSb.