Platinum (Pt)-based nanozymes display exceptional stability and catalytic activity in the activation of HO, making them ideal peroxidase (POD)-like substitutes for immunoassay applications. However, specific catalytic progress is hindered by the excessive orbital overlaps between Pt and oxygen-based intermediates. Herein, a highly efficient mesoporous medium-entropy alloy (m-MEA) nanozyme is reported to selectively enhance POD activity through synergy interaction of multiple elements. Such synergy reduces the 5 orbital energy of Pt, thereby lowering antibonding energy levels and weakening - orbital hybridization between the O 2 and Pt 5. The reduced orbital overlap lowers energetic barriers and suppresses the excessive adsorption of oxygenated intermediates (*OH/*OH), as well as weakens oxygen poisoning of active sites. In addition, the exposed mesoporous structure of m-MEA nanozyme ensures accessible active sites, resulting in a high POD specific activity of 304 ± 1.69 U mg, which is 6.58- and 507-folds higher than that of mesoporous Pt and nonporous MEA nanozymes, respectively. The m-MEA-based immunoassay platform has been utilized for the detection of prostate-specific antigens, achieving an exceptionally low detection limit of 1.20 pg mL, surpassing the sensitivity of traditional enzyme-linked assays.