The sulfide compound Bi2SeS2is an environmentally friendly thermoelectric material, which is as an analogy of the traditional compound Bi2S3. Here, a high ZTvalue of 1.04 at 773 K is obtained through synergistically tuning the electrical transport properties by using Cu and I as the dual carrier providers and suppressing the lattice thermal conductivity by forming the Cu-rich nano-precipitates. The dual carrier providers lead to exclusively high electrical conductivity and hence a large power factor over 650μWm−1K−2at 773 K. The atomic resolved annular dark field STEM reveals the interstitial Cu and S deficiency would result into the formation of BiCuS2or Cu2S nano-precipitates and embed in the lattice of Bi2SeS2, serving as strong phonon scattering. This work suggested that nanostructured Bi2SeS2is very promising for waste heat energy harvesting in the future.