Due to conflicts in physical mechanisms, simultaneously improving the dielectric constant (εˊ) and breakdown field strength (E_b) of dielectric materials remains challenging. Fortunately, progress has been made in addressing this issue, though the underlying mechanisms still require further investigation. In this study, Ca_1–xNa_xCu_2.95Ni_0.05Ti₄O₁₂ (x = 0, 0.003, 0.005, 0.007) ceramics were prepared, and the sample with x = 0.005 exhibited significant enhancement in both εˊ and E_b. Through density functional theory calculations and X-ray photoelectron spectroscopy, Na doping is confirmed to promote Cu⁺ and V_O^‥ concentration, while Ni increases Ti³⁺ content. Notably, Cu⁺ and Ti³⁺ together enhance εˊ, and the increased V_O^‥ forms depletion layers to improve E_b. These findings clarify the synergistic mechanism of Na–Ni co-doping for the simultaneous improvement of εˊ and E_b in CCTO ceramics. They not only provide research insights for the composition design of high performance ceramic capacitors but also hold great promise for the development of high energy density dielectric ceramics.