The invention discloses a corrosion inhibitor capable of inhibiting carbon steel from corroding in salt water (sea) medium, a preparation method and application thereof, and belongs to the technical field of corrosion inhibition of carbon steel. The corrosion inhibitor comprises 20-40 parts of zinc sulfate, 5-15 parts of calcium gluconate, 5-15 parts of sodium tungstate, 3-10 parts of sodium silicate, 1-3 parts of thiourea, 1-5 parts of ethanolamine phosphate, 0.3-0.8 parts of dodecyl polyglucoside, 0.5-1 part of hexadecanoic acid isopropyl ester, and 30-45 parts of water. The preparation method comprises the following steps: (a) dissolving the calcium gluconate in water according to the proportion, stirring and dissolving; (b) adding the sodium tungstate into the solution obtained by the step (a), stirring and dissolving; (c) sequentially adding zinc sulfate, thiourea, ethanolamine phosphate and dodecyl polyglucoside into the solution obtained by the step (b) according to the proportion, stirring and dissolving, and adding calcium silicate and stirring; (d) adding hexadecanoic acid isopropyl ester according to the proportion and stirring for 10-15 min. The corrosion inhibitor has advantages of high corrosion inhibition efficiency, small dosage, low cost, convenient use, and environmentally friendly type.