AbstractPolyethylene oxide (PEO)‐based solid polymer electrolytes are regarded as promising electrolyte materials because of their safety and flexibility. However, low ionic conductivity at ambient temperature and poor mechanical properties have been a hindrance to its development. In this work, we thermally compressed the PEO‐based electrolytes and explored the ionic conductivity, mechanical performance, free volume, and thermal properties of the electrolyte under different thermal pre‐compressing strains (TPC‐strains). The results show that TPC‐strain can significantly improve the ionic conductivity, in‐plane strength, stiffness, and cell specific capacity as well as the mechanical integrity of solid polymer electrolytes within the battery environment. However, it also results in a reduction in the modulus and stiffness of the SPEs in the through‐plane. In particular, applying a TPC‐strain of 10%–20% to the SPEs by thermal compressing may be a suitable option, which can increase the ionic conductivity of the through‐plane to a factor of 3.4 compared with the uncompressed electrolyte, and increase the in‐plane strength by up to 141%, resulting in better mechanical integrity during charging/discharging processes. At the same time, the compression modulus can be maintained at 80% or higher.