The sodium super-ionic conductor (NASICON)-type solid-state electrolyte Li1.4Al0.4Ti1.6(PO4)3 (LATP) is an attractive alternative to liquid electrolytes for lithium batteries. The rapid development of LATP, however, is hindered by its poor interfacial compatibilities against the Li metal. Herein, a flexible membrane coating layer consisting of Mg3N2 and PVDF has been adopted to modify LATP via a simple drop-casting method. A multifunctional interlayer with Mg, LiF and Li3N is in situ constructed by the reaction of the coating layer with the Li metal. The decomposition of LATP has been restrained and interfacial ionic transport kinetics has been improved with the modification. Benefitting from the multifunctional interlayer, the critical current density of LATP is improved from 0.34 mA cm−2 to 0.76 mA cm−2. The symmetric cells assembled with the modified LATP exhibit a stable cycle for more than 1000 h at 0.20 mA cm−2, and the Li/LiFePO4 cells after modification have a capacity retention of 80% after 385 cycles at 2C. The present work demonstrates a promising strategy for fine interfacial stability tuning and low-impedance LATP.