Due to the limited oxidation stability (<4 V) of ether oxygen in its polymer structure, polyethylene oxide (PEO)‐based polymer electrolytes are not compatible with high‐voltage (>4 V) cathodes, thus hinder further increases in the energy density of lithium (Li) metal batteries (LMBs). Here, a new type of polymer‐in‐“quasi‐ionic liquid” electrolyte is designed, which reduces the electron density on ethereal oxygens in PEO and ether solvent molecules, induces the formation of stable interfacial layers on both surfaces of the LiNi1/3Mn1/3Co1/3O2 (NMC) cathode and the Li metal anode in Li||NMC batteries, and results in a capacity retention of 88.4%, 86.7%, and 79.2% after 300 cycles with a charge cutoff voltage of 4.2, 4.3, and 4.4 V for the LMBs, respectively. Therefore, the use of “quasi‐ionic liquids” is a promising approach to design new polymer electrolytes for high‐voltage and high‐specific‐energy LMBs. A new type of polymer‐in‐“quasi‐ionic liquid” electrolyte (PQILE) for high‐voltage and long‐term cycling of Li metal batteries is reported. The donation of lone electron pairs of ether oxygen atoms to Li+ cations greatly improves the oxidation stability of PQILE. Li||LiNi1/3Mn1/3Co1/3O2 batteries with optimized PQILE can be stably cycled at 4.4 V for 300 times with a capacity retention of 79.2%.