Mechanical relaxation behavior of high-entropy metallic glasses is not only crucial for understanding the glass transition phenomenon, plastic deformation and relaxation mechanism but also help for extending their application in engineering. The mechanical relaxation behavior of Ti₂₀Zr₂₀Hf₂₀Cu₂₀Be₂₀ high-entropy metallic glass (HE-MG) was investigated by static stress relaxation analysis in the current research. With a constant strain, the stress relaxation process was analyzed in a wide time window and temperature range. A decoupling of the relaxation into a two-step (slow relaxation and fast relaxation) was found in the glass state below T_g. The slow relaxation process exhibits a stretched decay and a significant Arrhenius dependence on the temperature. The relaxation time decreases sharply with the increase of temperature, which may relate to the long-range atomic rearrangements at larger scales. The fast relaxation relates to the atomic scale internal stress dissipation. In addition, the strain has no distinct influence on the stress relaxation process, whether high-entropy metallic glass is deformed at the elastic stage or yields. This study reveals the decoupling phenomenon and the related unique dynamics relaxation mechanism in high-entropy metallic glass, which will enhance our understanding of the mechanical relaxation behavior and intrinsic characteristics of high-entropy metallic glasses.