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.