AbstractIn situ polymerization to prepare quasi‐solid electrolyte has attracted wide attentions for its advantage in achieving intimate electrode–electrolyte contact and the high process compatibility with current liquid batteries; however, gases can be generated during polymerization process and remained in the final electrolyte, severely impairing the electrolyte uniformity and electrochemical performance. In this work, an in situ polymerized poly(vinylene carbonate)‐based quasi‐solid electrolyte for high‐voltage sodium metal batteries (SMBs) is demonstrated, which contains a novel multifunctional additive N‐methyl‐N‐(trimethylsilyl)trifluoroacetamide (MSTFA). MSTFA as high‐efficient plasticizer diminishes residual gases in electrolyte after polymerization; the softer and homogeneous electrolyte enables much faster ionic conduction. The HF/H₂O scavenge effect of MSTFA mitigates the corrosion of free acid to cathode and interfacial passivating layers, enhancing the cycle stability under high voltage. As a result, the 4.4 V Na||Na₃V₂(PO₄)₂F₃ cell employing the optimized electrolyte possesses an initial discharge capacity of 112.0 mAh g⁻¹ and a capacity retention of 91.3% after 100 cycles at 0.5C, obviously better than those of its counterparts without MSTFA addition. This work gives a pioneering study on the gas residue phenomenon in in situ polymerized electrolytes, and introduces a novel multifunctional silane additive that effectively enhances electrochemical performance in high‐voltage SMBs, showing practical application significance.