The increasing electromagnetic pollution necessitates the development of advanced microwave absorbers. Although MAX phases exhibit chemical stability and electrical conductivity, their absorption performance is limited by a singular loss mechanism. Here, we propose a “pre-placed vacancy and isomorphous occupancy” strategy to engineer A-site high-entropy (HE) MAX phases, achieving unprecedented incorporation of large-radius elements (Ag and Bi). The optimized absorber delivers exceptional microwave absorption performance, with a minimum reflection loss of −71.6 dB (at 3.05 mm) and a broad effective absorption bandwidth of 4.1 GHz (at just 1.25 mm), outperforming both reported MAX phase variants and commercial absorbers. These remarkable properties stem from three synergistic mechanisms: A-site composition tailoring optimized impedance matching, HE-induced lattice distortion enhanced dipolar polarization, and A-site entropy engineering increased conduction loss. Our work pioneers a novel method for manipulating electromagnetic response in MAX phases through atomic-scale entropy engineering, paving the way for next-generation electromagnetic protection materials.