The invention relates to the field of buildings, in particular to an admixture for marine engineering concrete and a preparation method of the admixture. The admixture is prepared from raw materials in parts by weight: 100-150 parts of a water reducing agent, 0.5-1 part of tetramethylammonium iodide, 1-5 parts of sodium ethoxide, 3-5 parts of polyacrylamide, 30-50 parts of a surfactant, 15-20 parts of a rust inhibitor, 30-50 parts of an expansion agent, 3-5 parts of a retarder, 6-10 parts of a deforming agent and 0-80 parts of water. The invention aims to overcome the shortcomings of the priorart and provide the admixture for the marine engineering concrete and the preparation method of the admixture, the effects of water reduction, rust resistance, defoaming and retarding are achieved, and the effects are improved; the admixture is added into the concrete, so that the flowability of concrete slurry is improved, retentivity is good, the water reduction rate is high, the density of theconcrete is improved, good flocculability is achieved, and the friction resistance between liquid can be reduced; and the grinding efficiency is improved, and the admixture has the characteristics ofexcellent anti-permeability, fast hardening and early strength, high later strength and the like.