Cu_1.8S-based materials have become potential thermoelectric materials due to their rich raw material reserves, low toxicity, and excellent electrical and thermal properties. In this study, a series of Cu_1.8–x Sb_x S (x = 0, 0.005, 0.02, 0.03, 0.04) bulk materials is synthesized by using mechanical alloying combined with spark plasma sintering process. This preparation method can shorten the preparation cycle of materials, and effectively improve the research and development efficiency of thermoelectric (TE) materials due to its simple process. The effects of different Sb doping amounts on the structure, micromorphology, and thermoelectric transport properties of Cu_1.8–x Sb_x S phase are investigated. The results show that when 0 ≤ x < 0.02, the bulk samples are single-phase Cu_1.8S. With the further increase of Sb doping to 0.02 ≤ x ≤ 0.04, the second phase CuSbS₂ is formed when Sb content exceeds the solid solubility limit of x = 0.02 in Cu_1.8S, all Cu_1.8–x Sb_x S bulk samples exhibit p-type conductivity characteristics. Benefitting from the synergistic phonon scattering effect by multiscale defects, such as point defects (\rmSb_\rmCu^ \bullet\bullet , \rmV_\rmS^ \bullet \bullet ), nanopores, secondary phases (CuSbS₂), and dislocations, the thermal conductivity κ declines significantly from 1.76 W·m^–1·K^–1 (x = 0) to 0.99 W·m^–1·K^–1 at 723 K for the Cu_1.76Sb_0.04S sample. Finally, the peak dimensionless TE figure of merit (ZT ) value of 0.37 is achieved at 723 K for Cu_1.77Sb_0.03S resulting from a low thermal conductivity of 1.11 W·m^–1·K^–1 combining an appropriate power factor of 563 μW·m^–1·K^–2, which is 12% higher than that (0.33) of pristine Cu_1.8S. Although the Sb doped Cu_1.8S-based samples have lower thermal conductivity κ, the reduced power factor cannot be offset by reducing the thermal conductivity κ, so the TE figure of merit (ZT ) value is not significantly improved. Therefore, there is still much room for improving the performance of Sb doped Cu_1.8S-based thermoelectric material, and its thermoelectric performance can be further optimized through nano-second phase recombination, energy band engineering, and introducing multi-scale defects, etc. Our results suggest that the introduction of Sb into thermoelectric materials is an effective and convenient strategy to improve ZT value by reducing thermal conductivity κ.