The invention discloses a preparation method of ultrathin porous Co nanosheets. The method comprises the following steps: taking cobalt chloride and potassium hexacyanocobaltate as precursors and preparing cyanogen glue; then taking sodium borohydride as a reducing agent; reducing the cyanogen glue into the ultrathin porous Co nanosheets with the regular shape and uniform size by adopting a method for heating in a water solution. The preparation method disclosed by the invention is simple and economical and is suitable for industrial large-scale production; the ultrathin porous Co nanosheets prepared by the preparation method have good stability and water solubility, have very good OER (Oxygen Evolution Reaction) catalytic activity and stability in an alkaline potassium hydroxide solution and have a very good application prospect in the aspect of electrochemistry.