The invention discloses a liquid compound trace element fertilizer and a preparation method thereof. The fertilizer comprises the following ingredients by weight percent: 0-1.0% of citric acid, 0-1.0% of ascorbic acid (vitamin C), 0-1.0% of hydroxylamine hydrochloride, 0-1.0% of sorbitol, 0-1.0% of xylitol, 0-1.0% of mannitol, 0.46-38% of ferrous sulfate heptahydrate, 0.13-24% of manganese sulfate monohydrate, 0.18-3.3% of copper sulfate pentahydrate, 0.25-2.3% of zinc sulphate monohydrate, 0.25-4% of disodium octaborate tetrahydrate, 0.08-1.5% of ammonium molybdate, and the balance of pure water, wherein the dosage of the citric acid, the dosage of the ascorbic acid and the dosage of the hydroxylamine hydrochloride cannot be 0 at the same time, and the dosage of sorbitol, the dosage of xylitol and the dosage of mannitol cannot be 0 at the same time. According to the invention, an antioxidant prepared from one, two or more of the citric acid, ascorbic acid and hydroxylamine hydrochloride can effectively inhibit ferrous ions from being hydrolyzed into ferric ions, so that the iron ion absorption efficiency is improved, and the color of the product does not change at a high temperature; the raw material resources are rich, the production cost is low, and the equipment and production technologies are simple.