The invention discloses an environmentally-friendly desizing agent used for high-density polyester fabric. The environmentally-friendly desizing agent is prepared from, by weight, 3-6 parts of sodium hydroxide, 2-5 parts of sodium percarbonate, 1-3 parts of sodium bromite, 1.5-3 parts of tetraacetyl ethylenediamine,5-10 parts of cellulose compound enzyme, 2-3 parts of calcium chloride, 1-4 parts of secondary alkyl sodium sulfonate, 5-8 parts of n-butyl acetate, 1-2.5 parts of alkali lignin, 1-2 parts of a buffering agent, 0.5-1.5 parts of a penetrating agent and 20-50 parts of deionized water. The invention further discloses a preparation method of the environmentally-friendly desizing agent. The desizing agent is good in desizing effect, excellent in acid and alkali resistance, free of damage to the fabric and small in usage quantity, cleaning is easy, the yield of waste water is low, and no pollution is generated; the preparation method is simple, the raw materials are easy to obtain, the price is low, and the production cost is low.