Abstract: The high-entropy strategy has recently gained significant attention as an effective approach for enhancing microwave absorption performance. In this paper, a simple hydrothermal method followed by calcination was used to synthesize (Fe_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2)Fe₂O₄ high-entropy spinel ferrite for microwave absorption. The effects of calcination temperature on the microstructure and microwave absorption performance of the material were systematically investigated. Phase analysis reveals that a pure spinel ferrite phase with microwave absorption capability forms at 400°C, while minor maghemite impurities emerge at 800°C. As temperature rises, the effective absorption bandwidth first increases and then decreases, with its frequency range shifting toward higher frequencies. The sample calcined at 400°C achieves an excellent reflection loss of −77.48 dB with an effective absorption bandwidth of 3.74 GHz. The sample optimized at 800 °C exhibits an excellent absorption bandwidth of 6.71 GHz (10.61–17.32 GHz), along with a minimum reflection loss of −31.17 dB at a thickness of only 2.19 mm. It also demonstrates a significant radar cross-section reduction of 26.04 dB·m². The excellent microwave absorption performance is primarily attributed to enhanced dipole polarization induced by the high-entropy effect, favorable electrical conductivity, effective magnetic loss, and adequate impedance matching. This work provides a good example for constructing high-performance spinel oxides through high-entropy strategies, demonstrating significant potential in the development of advanced microwave absorption materials.