Abstract: Boundary engineering has proven effective in enhancing the thermoelectric performance of materials. SnSe, known for its low thermal conductivity, has garnered significant interest; however, its application is hindered by poor electrical conductivity. Herein, the Ag₈GeSe₆ is introduced into the p-type polycrystalline SnSe matrix to optimize the thermoelectric performance, and the in-situ Ag₂Se precipitates are formed in grain boundaries, which play dual roles, acting as an electron attraction center for improving hole concentration and a phonon scattering center for reducing lattice thermal conductivity. It effectively decouples the thermal and electrical transport properties to optimize the thermoelectric performance. Importantly, the amount of Ag₂Se can be controlled by adjusting the amount of Ag₈GeSe₆ added to the SnSe matrix. The introduction of Ag₈GeSe₆ enhances electrical conductivity due to the increased hole carrier caused by the introduced Ag⁺ and the formed electron attraction center (in-situ Ag₂Se precipitates). Based on the DFT calculations, the band gap of the Ag₈GeSe₆-doped samples is considerably decreased, facilitating carrier transport. As a result, the electrical transport properties increase to 808 µW m^-1 K^-2 at 823 K for SnSe + 0.5 wt% Ag₈GeSe₆. In addition, in-situ Ag₂Se precipitates in grain boundaries strongly enhance phonon scattering, causing a decrease in lattice thermal conductivity. Furthermore, the presence of defects contributes to a reduction in lattice thermal conductivity. Specifically, the thermal conductivity of SnSe + 1.0 wt% Ag₈GeSe₆ decreases to 0.29 W m^-1 K^-1 at 823 K. Consequently, SnSe + 0.5 wt% Ag₈GeSe₆ obtains a high ZT value of 1.7 at 823 K and maintains a high average ZT value of 0.57 over the temperature range of 323-773 K. Additionally, the mechanical properties of Ag₈GeSe₆-doped also show an improvement. These advancements can be applied to energy supply applications during deep space exploration.