O3-type layered metal oxides have excellent prospects for cathode materials in Na-ion batteries (NIBs) on account of their advantages, such as low cost. The layered oxide cathodes are now promising for commercial utilization. Nevertheless, the serious and irreversible phase transformation at high voltages, along with the slow kinetics of Na⁺ ion diffusion, presents considerable obstacles to the attainment of high-performance layered cathodes. In this work, we develop a novel layered cathode, O3-NaNi_0.25Fe_0.2Mn_0.3(LiCuAlTiSn)_0.05O₂ (HEO-NFM), based on the tiny high-entropy substitution strategy. The material with tiny high entropy substitution is proven to maintain the original crystal structures. The phase transition properties between P3 and O3 were improved, and the ordered arrangement of Na vacancies was significantly reduced. The structural entropy is greatly increased by introducing multiple elements into NaNi_1/3Fe_1/3Mn_1/3O₂ (NFM) transition metal layers, hence enhancing structural stability in the electrochemical reactions. HEO-NFM shows outstanding rate capability (106 mAh·g⁻¹ at 10 C) along with ultra-high cycling stability (70% capacity retention after 500 cycles). The full battery provides a capacity of 97 mAh·g⁻¹ at 5 C and maintains 81.4% capacity retention after 200 cycles, indicating great potential for applications. Herein, the tiny high-entropy substitution strategy inspires more rational designs of cathode materials with superior performances.