The invention discloses an additive premix for mixed culture of Litopenaeus vannamei and Siganus oramin and a preparation method of the additive premix. The additive premix is prepared from multivitamins and complex mineral salts, wherein the multivitamins are prepared from multiplex vitamins, DL-calcium pantothenate, nicotinic acid, folic acid, inositol, biotin, vitamin C, carnitine, choline chloride, bacillus subtillis, mannan, cholesterin, an antioxidant, a mildew preventive, and defatted rice bran; the complex mineral salts are prepared from cobalt chloride, copper sulfate pentahydrate, ferric citrate, magnesium sulfate, manganese glycinate, potassium iodide, sodium selenite, zinc sulphate monohydrate, sodium chloride, sodium dihydrogen phosphate, calcium lactate, ammonium molybdate, monocalcium phosphate, monopotassium phosphate and zeolite powder. The additive premix is added with the multiple components to meet the requirements of the Litopenaeus vannamei and the Siganus oramin for microconstituents such as the vitamins, mineral substances and the like in a mixed culture mode, has the high cost performance of a feed, and makes up the deficiency on nutrition research on the mixed culture of the Litopenaeus vannamei and the Siganus oramin.