High-entropy oxides (HEOs) composed of multiple metal elements have garnered significant attention as anode materials for lithium-ion batteries (LIBs), owing to their synergistic effects between constituent metal oxides and broad material design flexibility. However, the advancement of HEOs in LIBs has been hindered by time-consuming synthesis methods, complex fabrication procedures, and an insufficient understanding of their lithium storage mechanisms. In this study, a rock-salt structure HEO Fe_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2O was ultrafast synthesized by the Joule heating technique within 3 s and was applied to LIBs for the first time as a conversion-type anode material. The material exhibits not only excellent capacity retention but also remarkable structural reversibility. Specifically, the reversible capacity is determined to be 1310 mAh/g for 200 cycles at 0.1 A/g, and 705 mAh/g for 3000 cycles at 5 A/g. Detailed mechanistic investigations reveal that ZnO serves as an electrochemically inactive structural stabilizer that maintains the rock-salt framework, while Cu²⁺ is difficult to oxidize back to its original state once reduced to Cu⁰. This study provides critical insights into the composition-structure-property relationships of HEOs, offering valuable guidance for designing high-performance LIBs anode materials through entropy engineering.