This work designs a new system (1-x)Na_0.5Bi_0.5+yTiO₃-xNaTaO₃ with a nonstoichiometric bismuth ratio, which is used as dielectrics of ceramics capacitors. Phase structure evolution of (1-x)Na_0.5BiTiO₃-xNaTaO₃ is characterized using XRD, Raman and TEM. Dielectric and resistant properties of (1-x)Na_0.5Bi_0.5+yTiO₃-xNaTaO₃ are investigated with increasing concentration of NaTaO₃. With these investigations, the structure and defect chemistry of (1-x)Na_0.5Bi_0.5+yTiO₃-xNaTaO₃ are rationalized and their respective impact on capacitor properties are elucidated. The optimized composition 0.8Na_0.5Bi_0.51TiO₃-0.2NaTaO₃ possesses an ultra-wide operating temperature range (-92–398 ℃), in which both stable permittivity and low dielectric loss is obtained. Furthermore, the fabrication of multilayer ceramics capacitors (MLCC) based on 0.8Na_0.5Bi_0.51TiO₃-0.2NaTaO₃ dielectrics is investigated. With the addition of sintering aids, 0.8Na_0.5Bi_0.51TiO₃-0.2NaTaO₃ could be co-fired with Ag at 910 ℃ in air, and the low temperature co-fired ceramic (LTCC) capacitors maintain good temperature stability of permittivity and low dielectric loss in -100–352 ℃. Therefore, our work provides a new route for preparing ultra-wide operating temperature capacitors at low manufacturing costs.