The disclosure discloses a method for preparing biochar from Enteromorpha prolifera and use of the biochar. The method includes: subjecting Enteromorpha prolifera to lyophilization, then impregnating lyophilized Enteromorpha prolifera with phosphoric acid, and drying to a constant weight; grinding a dried mixture in a mortar, and then subjecting a ground sample to activation treatment in a tube furnace at 400° C. to 1,000° C. under a nitrogen atmosphere; and boiling an obtained solid product with concentrated hydrochloric acid, then washing with ultrapure water, and drying to a constant weight to obtain Enteromorpha prolifera biochar. The biochar is used in adsorption of cadmium. The method for removing cadmium using Enteromorpha prolifera biochar provided by the disclosure exhibits an extremely high adsorption and removal capacity for cadmium, with an adsorption capacity of biochar up to 250 mg/g or more.