High-entropy oxides (HEO) are a novel material with the unique electrochemical properties, which are stabilized due to the high configurational entropy. In this paper, a high-entropy (Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2)O nanopowder with the sizes of 40–65 nm and a rocksalt structure was synthesized by a polyacrylamide gel method and subsequent calcination at 900 ℃ for 2 h in the presence of a mole ratio of acrylamide/metal cations of 120:1. The results show that the temperature of forming a single-phase (Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2)O nanopowder decreases with the increase of the mole ratio of acrylamide/metal cations. The HEO nanopowder has a specific capacitance of 402 F/g at a current density of 1 A/g. A rate capability of 62% appears at 20 A/g. The capacitance retention is 61% after 2000 cycles at a current density of 5 A/g. It is indicted that (Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2)O nanopowder could be used as a prospective electrode material for supercapacitors.