<p>Deep cryogenic treatment (DCT) is widely recognized as an effective method to enhance the properties of metallic materials. At present, the impact mechanism of DCT on the corrosion behavior of CoCrFeMnNi high-entropy alloy (HEA) with excellent comprehensive performances remains unstudied. This study aims to explore the influences of DCT and cyclic deep cryogenic treatment (CDCT) on the microstructure, hardness and anti-corrosion ability of CoCrFeMnNi HEA in simulated oceanic environment utilizing electrochemical tests, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and immersion experiment. Results indicate that the thick and uniform passive film of CDCTed HEA exhibits the optimal protective performance, compactness and stability, as verified by the elevated passive film resistance (<i>R</i>_f) of 168.3 kΩ·cm², the decreased passivation film capacitance (<i>Q</i>_<i>f</i>) of 3.231 × 10⁻⁵Ω⁻¹ ·&#xa0;cm⁻²&#xa0;· s⁻ⁿ and carrier density, which is attributed to increased corrosion-resistant oxides, bound water content and less instable manganese oxides, all of which synergistically inhibits the penetration of corrosive ions. The reduced quantity of metastable and stable pits, and higher pitting potential (<i>E</i>_p) of 0.161&#xa0;V reveal that DCT remarkably improves the pitting resistance of the HEA. Specifically, the alloy prepared by CDCT demonstrates superior anti-corrosion property. Moreover, DCT results in a reduction in grain size, while CDCT further induces the grain refinement. Additionally, the HEA subjected to CDCT possesses the highest Vickers hardness value.</p> Graphical abstract <p>This study investigated the microstructure and corrosion characteristics of CoCrFeMnNi HEA after deep cryogenic treatment and cycle deep cryogenic treatment.</p>