Provided are a cleaning agent and a preparation method and the use thereof. The cleaning agent is prepared from the following raw materials comprising the following mass fraction of components: 0.5%-20% of an oxidant containing iodine, 0.5%-20% of an etchant containing boron, 1%-50% of a pyrrolidinone solvent, 1%-20% of a corrosion inhibitor, 0.01%-5% of a metal ion-free surfactant, and water, with the sum of the mass fraction of each component being 100%, the pH of the cleaning agent is 7.5-13.5, and the corrosion inhibitor is one or more of a benzotriazole corrosion inhibitor, a hydrazone corrosion inhibitor, a carbazone corrosion inhibitor and a thiocarbohydrazone corrosion inhibitor. The cleaning agent can efficiently remove nitrides from hard mask residues with little effects on metals and low-κ dielectric materials, and has a good selectivity.