[en] In the present study, series of garnet-type Li_6.75+xLa_3-xSr_xZr_1.75Nb_0.25O₁₂ solid electrolytes [LLSZN with various Sr contents (x = 0.05-0.25)] have been prepared via conventional solid-state method. The effects of Sr contents on their phase structure and ionic conductivity have been systematically investigated on the combined measurements of X-ray diffraction and scanning electron microscopy and alter current impedance spectroscopy. Our results reveal that a phase transition from tetragonal to cubic structure occurs when both Sr and Nb elements is introduced, and such a cubic structure can be stable over the whole Sr contents variation, which is suggested to provide a beneficial impact on the performance of LLSZN. Accordingly, both relative density and total ionic conductivity exhibit a favorable tendency of increasing first and then decreasing with increased Sr contents, wherein a peak value at 93.46% and 5.09 x 10^-4 S cm^-1, respectively, can be well achieved. Particularly, the maximum ionic conductivity is almost twice that of the compared sample (2.93 x 10^-4 S cm^-1), and possess the minimum activation energy ∝ 0.30 eV. Such a modification method, featured with higher efficiency and lower cost, is expected to be helpful for the development of solid electrolyte. (orig.)