The invention belongs to the technical field of water treatment, and particularly relates to an inorganic water-soluble gel corrosion inhibitor used for controlling the corrosion of carbon steel of a circulating water system; the corrosion inhibitor comprises the following components according to mass ratio: 20%-43% of sodium pentaborate, 6%-18% of sodium tetraborate, 8%-22% of boric acid, 1%-10% of molybdate, 0.3%-1% of zinc salt, and the balance of deionized water. According to the water-soluble gel used for controlling the corrosion of carbon steel of the circulating water system, borate, molybdate, zinc salt and deionized water are mixed in different proportion, so as to prepare a water soluble gel-state compound, the high efficiency synergistic effect among the raw materials is utilized, and the inhibition efficiency for carbon steel can reach 95% or above; besides, chromate, phosphate and nitrite are not used, and the chemical can be conveniently conveyed in the circulating water system.