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Optimizing the performance of thermoelectric materials by reducing its thermal conductivity is crucial to enhance its thermoelectric efficiency. Novel thermoelectric materials like the CuGaTe2 compound are hindered by high intrinsic thermal conductivity, which negatively impacts its thermoelectric performance. In this paper, we report that the introduction of AgCl by the solid-phase melting method will influence the thermal conductivity of CuGaTe2. The generated multiple scattering mechanisms are expected to reduce the lattice thermal conductivity while maintaining sufficient good electrical properties. The experimental results were supported by first-principles calculations confirming that the doping of the Ag will decrease the elastic constants, bulk modulus, and shear modulus of CuGaTe2, which makes the mean sound velocity and Debye temperature of Ag-doped samples lower than those of CuGaTe2, indicating the lower lattice thermal conductivity. In addition, the Cl elements within the CuGaTe2 matrix escaping during the sintering process will create holes of various sizes within the sample. These combined effects of holes and impurities will induce phonon scattering, which further reduces the lattice thermal conductivity. Based on our findings, we conclude that the introduction of AgCl into CuGaTe2 has shown a lower thermal conductivity without compromising the electrical performance, resulting in an ultra-high ZT value of 1.4 in the (CuGaTe2)0.96(AgCl)0.04 sample at 823 K.