The crystal structure, electronic structure, microstructure, and terahertz dielectric performances of Zn1-xCuxTiNb2O8 (x = 0.000, 0.005, 0.020, and 0.050) ceramics prepared by solid-state reaction were investigated by XRD refinement, First-principle calculations, X-ray photoelectron spectroscopy, scanning electron microscope, and terahertz time-domain spectroscopy. According to electronic structure calculations, Nb–O bond plays a crucial role in terahertz dielectric performances. Combining the XPS results and the terahertz dielectric performances, it can be seen that after doping Cu2+ ions in Zn1-xCuxTiNb2O8 (0.000 ≤ x ≤ 0.050) ceramics, the reduction of Ti4+ ions is prevented by a favorable compensation mechanism, which reduces the dielectric loss of the specimen. However, excess Cu2+ ions lead to abnormal grain growth and increased dielectric loss. When sintered at a temperature of 1140 °C, Zn1-xCuxTiNb2O8 (x = 0.005) ceramics exhibit excellent terahertz dielectric performances (with a dielectric loss of about 0.08), which makes them promising for use in modern terahertz devices.