MgNb₂O₆ ceramics offer advantages such as moderate sintering temperature and low dielectric loss, making them widely applicable in the field of wireless communications. However, the correlation mechanism between MgNb₂O₆ structure and terahertz dielectric properties remains inadequately understood. To address this gap, MgNb_2-x(Ti_0.5W_0.5)_xO₆ (x=0-0.03) ceramics were prepared by solid state reaction method in this study. The crystal structure of MgNb_2-x(Ti_0.5W_0.5)_xO₆ (x=0-0.03) ceramics and its correlation with terahertz dielectric properties were studied using Rietveld refinement, complex chemical bond theory, and terahertz time domain spectroscopy. The global instability index was introduced to characterize the in-lattice strain in single-phase MgNb₂O₆ system, establishing a correlation among in-lattice strain, lattice energy and terahertz dielectric loss. The results show that (Ti_0.5W_0.5)⁵⁺ ions can modify the crystal structure. With the increase in doping amount, the internal strain reduces, and the stability of crystal structure and lattice energy increases, leading to decrease in dielectric loss. In addition, the atomic packing density increases with higher (Ti_0.5W_0.5)⁵⁺ ion doping, which effectively inhibits the anharmonic vibration of the crystal and further reduces the dielectric loss. At a sintering temperature of 1340 ℃, MgNb_2-x(Ti_0.5W_0.5)_xO₆ (x=0.03) ceramics have excellent terahertz dielectric properties, with a dielectric constant (ε_r) of 19.32, a dielectric loss of 0.003 (@0.30 THz), and an absorption coefficient of 1.64 cm^-1 (@0.30 THz). The MgNb_2-x(Ti_0.5W_0.5)_xO₆ (x=0.03) ceramics prepared in this study have a good application prospect in terahertz devices.