It is of great importance to discover completely the correlation between the defects and positron lifetimes in positron annihilation technology (PAT) and reveal clearly the effect of defects on the dielectric constant of universal microwave ceramics. In this work, ZnNb2−xTixO6−x (0 ≤ x ≤ 0.5) dielectric ceramics with nonstoichiometric defects were prepared by solid-state reaction. All the pores and defects in as-prepared ceramics were accurately revealed by PAT as a convenient way. The positron lifetimes of τ3 and τ2 in positron annihilation lifetime spectra (PALS) were dependent on two kinds of pores correlated with where o-Ps atoms and trapping positron annihilated in, whereas τ1 could be separated evidently into τb and τt which are related to bulk structure and negatively charged defects in the system. The dielectric constant of microwave ceramics ZnNb2−xTixO6−x, which increases from 26 to 49 with x increasing from 0 to 0.5 at 10 GHz, is confirmed to be directly controlled by the negatively charged defect of through cation rattling.