Gel polymer electrolytes (GPEs) with flexibility, easy processability, and low cost have been regarded as promising alternatives for conventional liquid electrolytes in next-generation sodium metal batteries (SMBs). However, GPEs often suffer from combustion risk and inferior interfacial compatibility toward Na metal anode, which severely limit their wide commercial applications. Here, a rational design of asymmetric fireproof GPE (AFGPE) modified with a boron-contained covalent organic framework (BCOF) on one side is developed through in-situ crosslinking polymerization process. Benefiting from the unique structure and composition, the resulting AFGPE exhibits high Na⁺ transference number, wide electrochemical window, excellent mechanical properties and high safety. Especially, the nanoscale BCOF layer with uniform nanochannels works as ion sieve that homogenizes Na⁺ flux during Na plating process, while the abundant Lewis-acid B sites can strongly capture counter anions and decrease space charge layer at anode side, thus promoting the uniform Na deposition to effectively suppress dendrite growth. Consequently, the Na/AFGPE/Na symmetric cells demonstrate remarkable cycling stability for over 1200 h at 0.1 mA·cm⁻², and the solid-state SMBs exhibit outstanding cycling properties and rate capability, delivering a high capacity retention of 96.4% under current density of 1 C for over 1000 cycles.