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Electromagnetic (EM) absorbers serving in the megahertz (MHz) band and a wide temperature range (from −50 to 150 °C) require high and temperature-stable permeability for outstanding EM absorption performance. Herein, FeCoNiCr0.4CuXhigh-entropy alloy (HEA) powders with a unique nanocrystalline structure separated by a thin amorphous layer (NTA) are designed to improve permeability and enhance intergranular coupling. Simultaneously, the long-range anisotropy is introduced via devising the preparation process and tuning the chemical composition, such that the intergranular exchange interaction is further strengthened for stable permeability and EM wave absorption in a wide temperature range. FeCoNiCr0.4Cu0.2HEAs exhibit a near-zero permeability temperature coefficient (5.7 × 10–7°C–1) a in wide temperature range. The maximum reflection loss (RL) of FeCoNiCr0.4Cu0.2HEAs is higher than −7 dB with 5 mm thickness at −50–150 °C, and the absorption bandwidth (RL < −7 dB) can almost cover 400–1000 MHz. Furthermore, FeCoNiCr0.4Cu0.2HEAs also have a high Curie temperature (770 °C) and distinguished oxidation resistance. The permeability temperature dependence of FeCoNiCr0.4CuXHEAs is investigated in-depth in light of the microstructural change induced by tuning the chemical composition, and a new inspiration is provided for the design of magnetic applications serving in wide temperature, such as transformers, sensors, and EM absorbers.