Solid sodium metal batteries (SSMBs) offer an alternative promising power source for electrochemical energy storage due to their high energy density and high safety. However, the inherent sodium dendrite growth and poor mechanical properties of electrolytes seriously limit their application. Herein, a network structure composed of polyethylene oxide-based composite polymer electrolyte (CPE) is designed with liquid metal nanoparticles (LM) for SSMBs, in which LM can move in the solid electrolyte with the electric field driven. This effect can facilitate the inactivated sodium return to the metal sodium anode, and alloy with dendrites at the same time, which is beneficial for inhibiting the growth of dendrites. The symmetric cell with the CPE containing LM achieves good cyclic stability of more than 1800 and 800 h at 0.1 and 0.2 mA cm−2, respectively. The energy density of the pouch battery can reach 230 Wh kg−1. In sum, LM presents great potential to be employed as a performance reinforcement filler for CPEs, which paves the way for achieving high-performance SSMBs.