The invention relates to a preparation method of a red mud sinking agent, which comprises the following steps of mixing acrylamide, acrylic acid and urea with deionized water in weight percent, cooling and regulating the pH value to end the material preparation; mixing the ingredients, chelating agent and chain transfer agent in a reaction kettle, sequentially adding a third section oxidant, a second section oxidant, a first section oxidant, a reducer and an auxiliary reducer in nitrogen introducing process, continuously introducing nitrogen, and than stopping nitrogen introduction, sealing, and carrying out insulation polymerization reaction; opening a reaction kettle bottom valve after being placed, pressing out a rubber block by compressed air and disintegrating the rubber block into colloidal particles by a granulator; and hydrolyzing the colloidal particles, and preparing a product through treatments of granulation, drying, crushing, screening and packaging. The aluminum oxide plant red mud sinking agent has the advantages of high molecular weight, fast dissolving speed, high hydrolysis degree and low cost.