SnS composed of low toxicity, low-cost and earth-abundant elements, has extensive attention in the field of thermoelectric research. The n-type SnS_1-xCl_x (x=0, 0.02, 0.03, 0.04, 0.05, 0.06) polycrystalline bulk thermoelectric samples were prepared by mechanical alloying (MA) combined with Spark Plasma Sintering (SPS). Effect of Cl^- amounts on the phase structure, microstructure and thermoelectric transport properties were systematically studied. Results show that introduction of Cl^- enhances electron concentration which makes intrinsic p-type SnS change to n-type. With the amount of Cl^- increasing, the Hall carrier concentration of n-type SnS semiconductor increases from 6.31×10¹⁴cm^-3 (x=0.03) to 7.27×10¹⁵cm^-3 (x=0.06) at room temperature. The maximum electrical conductivity of 408 S•m^-1 the relatively high Seebeck coefficient of -553 μV•K^-1 are obtained at 823 K for x=0.05 sample, which produces the maximum power factor of 1.2 μW·cm^-1·K^-2. Addition of Cl^- can introduce point defects to scatter phonons, which makes the lattice thermal conductivity reduce from 0.67 W·m^-1·K^-1 (x=0) to 0.5 W·m^-1·K^-1 (x=0.02). The highest ZT~0.17 is obtained at 823 K for x=0.04 sample, which is 70% higher than that (ZT~0.1) of the pristine SnS.