Sodium metal batteries (SMBs) have attracted considerable attention owing to their dazzling advantages. Nevertheless, the brittle natural SEI film and the growth of dendrites have posed a great threat to the practical application of SMBs. Herein, an interphase layer (Ga/Na2Se) with a self-healing function assisted by liquid gallium metal is designed. By combining in situ characterizations and theoretical analysis, the operating mechanism of the interphase is comprehensively revealed. The in situ formed Ga liquid metal not only possesses a self-healing function, thus repairing the collapse of interfacial texture upon cycling, but also has a favorable Na+ diffusion barrier that endows Na+ to disperse uniformly instead of forming Na clusters on Na substrate, preventing uneven charge accumulation and preferential Na nucleation. Meanwhile, the Na2Se superionic conductor teases the Na+ flux on the interlayer, efficiently avoiding the formation of dendrites. Consequently, the symmetric cells achieve a superior cycling lifespan of 2200 h at 0.5 mA cm−2/1.0 mAh cm−2. Most importantly, the pouch full cell assembled using NaNi1/3Fe1/3Mn1/3O2 cathode also exhibits overwhelming cycling stability, delivering 75.9 mAh g−1 after 5000 cycles at 1.0 C. This work provides a new insight into using liquid metal for the practical exploitation of SMBs.