 A macromolecular electrolyte is designed with different chemical moieties to perform different functions, and it is extremely suitable to be applied in solid high-temperature lithium ion batteries. The preparation of the electrolyte involves synthesis of a comb-like macromolecule and immobilization of lithium ions. The comb-like macromolecule is synthesized via a ring-opening metathesis copolymerization of norbornene derivatives anchored with phosphate and polyethylene glycol monomethyl ether (–O(CH2CH2O)nCH3) and is partially cross-linked through polyethylene glycol (–(CH2CH2O)n–) bridge. The immobilization of lithium ions is carried out by lithium bis(trifluoromethyl-sulfonyl)imide. The as-prepared electrolyte membrane has a bicontinuous morphology consisting of a cross-linked mechanical scaffold intertwined with continuous Li+ ions conducting channels. Apart from lithium ion transference number close to unity and remarkable ionic conductivity, the electrolyte also displays strong strength, high flexibility, good thermal stability and outstanding flame retardancy. At 80 °C, LiFePO4/macromolecular electrolyte/Li coin cells, with the discharge capacity of 134.1 mAh g−1 at a current density of 0.2 C, are able to maintain a value of 120.7 mAh g−1 after 100 cycles of charge and discharge.