The invention discloses a preparation method of an alkali oxygen evolution reaction electrocatalyst. The preparation method comprises the following steps: firstly, carrying out ultrasonic cleaning on a conductive substrate, then preparing aqueous solution with soluble cobalt salt, soluble manganese salt, ammonium fluoride and urea, and in a reaction kettle, vertically growing a manganese cobalt subcarbonate nano array multilevel structure on the surface of the substrate; then preparing aqueous solution with soluble alkali and a reducing agent, carrying out secondary treatment, and carrying out structure and performance optimization on the manganese cobalt subcarbonate nano array multilevel structure; finally, in a tube furnace, in nitrogen or argon atmosphere, carrying out calcination at a temperature of 200 to 1,000 DEG C to prepare the alkali oxygen evolution reaction electrocatalyst with a manganese-doped cobalt oxide nano array multilevel structure. The preparation method disclosed by the invention adopts a simple hydro-thermal synthesis/calcination treatment method, is simple in process and is easy to regulate and control; the prepared product is excellent in performance and is an electrocatalyst with wide prospect in the application process of alkali water decomposition.