Dense (Zn0.5W0.5)xTi1-xO2 (ZWTOx) ceramics were fabricated using a conventional solid state reaction method with sintering under a nitrogen atmosphere (ZWTOx-N2) and an oxygen atmosphere (ZWTOx-O2), respectively. Colossal permittivity (ε > 104) and low loss (tan δ < 0.1) were simultaneously achieved in ZWTOx-N2 ceramics, and two types of dielectric relaxation behaviors observed were interpreted to be due to interface polarization and disassociation between oxygen vacancies and trivalent titanium ions, respectively. The impedance plots suggested that the ZWTOx-N2 ceramics are electrical heterostructures composed of semiconductor and insulator grain boundaries, which proved that the CP performance of ZWTOx-N2 ceramics almost originates from the internal barrier layer capacitance (IBLC) effect. In addition, a series of anomalous dielectric behaviors such as low permittivity and low frequency dispersion were observed for ZWTOx-O2 ceramics; polarization (P)-electric field (E) hysteresis loop curves were obtained for ZWTOx-O2 ceramics, and that impedance plots have shown that the ZWTOx-O2 ceramics display higher insulation resistivity. Density functional theory (DFT) calculations illustrated that the Zn2+-W6+ ion pairs are easy to form in ZWTOx-O2 ceramics, which causes destruction of the local lattice and thus leads to abnormal dielectric behavior. This work will provide a new strategy for defect engineering in TiO2 and other CP materials.