Cu nano/microparticle pastes have garnered considerable research interest and investigation for their application in the packaging of power devices. However, their inherent porosity negatively affects joint performance. In this study, we addressed the issue of porosity in Cu-Cu joints by incorporating a liquid metal (LM) into a Cu nano/microparticle paste solder. The LM exhibits wettability and flowability, enabling it to spontaneously fill the pores in the Cu nano/microparticle structure and establish connections with the substrate surface. Moreover, the LM creates additional conductive pathways and reduces thermal resistance, resulting in enhanced electrical and thermal conductivities. The experimental thermal conductivity reached an impressive value of 342.68 W/(m·K), while the observed electrical resistivity was as low as 6.16 μΩ·cm. These findings demonstrate the positive effect of LM on Cu-Cu joints and offer a novel theoretical foundation for improving the electrical and mechanical properties of such joints.