Polyethylene oxide (PEO)-based solid electrolytes, which exhibit ideal extensibility and wide electrochemical window, are considered as one of the most promising candidates for all solid-state sodium batteries (ASSSBs). However, the low mechanical strength and low ionic conductivity hinder their application. Herein, electrospun MgAl 2 O 4 nanofibers are complexed with PEO/NaClO 4 to enhance the mechanical and thermal stability. Determined by 23 Na solid-state nuclear magnetic resonance spectroscopy combined with first-principle calculations, the adsorption energy of ClO 4 − on the MgAl 2 O 4 surface is far higher than that on PEO, which facilitates the dissociation of Na + and ClO 4 − , thereby enabling a fast transport of Na + by the introduction of MgAl 2 O 4 in the polymer electrolyte. The ionic conductivity is enhanced to 1.89 × 10 −4 S cm −1 from 8.46 × 10 −5 S cm −1 at 55 °C, and the Na + transfer number is improved to 0.55 from 0.26 in the composite electrolyte. The Na//(PEO/MgAl 2 O 4 /NaClO 4 )//Na symmetric cell can cycle for over 400 h at a current density of 0.05 mA cm −2 and a cut-off capacity of 0.05 mAh cm −2 while the ASSSBs incorporating the polymer composite electrolyte also exhibit notable rate and cycle performances.