The effective, stable, and secure catalysts are essential for sulfate radical (SO₄^·−)-based advanced oxidation processes (SR-AOPs) to the degradation of organic contaminants in water. Heterogeneous supported cobalt-based catalysts are commonly used to activate peroxymonosulfate (PMS) to achieve the degradation. In this work, we synthesized Co₃O₄@Al₂O₃ three-dimensional (3D) mesoporous nanocomposite (denoted as Co₃O₄@Al₂O₃ 3DPNC) in just one step by calcining cheap and green deep eutectic solvent (DES) solution containing Co salt. Co₃O₄@Al₂O₃ 3DPNC with the high specific surface area (93.246 m²/g), uniform pore distribution (3.829 nm) and rich porosity (0.255 cm³/g) were attained in a beautiful hierarchical structure which exhibited the open 3D propeller-like microstructure, two-dimensional lamellar substructure with rich folds, as well as the decoration of highly dispersed Co₃O₄ nanoparticles on mesoporous amorphous Al₂O₃. The excellent chemical and thermal stability of Al₂O₃ ensures the high stability of the catalyst, and the formation of the complex hierarchical structure makes the active Co₃O₄ be homogenously dispersed for effective catalysis. The catalyst demonstrated outstanding performance for catalytic degradations of organic pollutants (acetaminophen, oxytetracycline, 5-sulfosalicylic acid, orange G and Rhodamine B) by generated SO₄^·−, ·OH and ¹O₂. With a very low cobalt content (equal to 28.2 mg/L of Co), the catalyst exhibited very high stability and excellent reusability in the recycling usages, while the leaching of the cobalt element (< 0.145 mg/L) was also at a low level. Our catalyst achieved effective degradations of acetaminophen in cycles without losing its stable hierarchical nanostructure.