The invention discloses a method for purifying kelp varieties in a sea area in a northern area. The method comprises the following steps: selecting at least 60 high-quality single plants in consistent form on the sea according to the original fine variety standard characteristics of kelps; analyzing and selecting a seaweed matched with an AFLP spectrum of a kelp gametophyte in a cultivation sea area as an object according to the molecular marker, and temporarily cultivating; after temporarily cultivating for a period of time, screening a main weed according to the variety characteristics; after cultivating the main weed till the sporangium is mature, releasing free spores; attaching the free spores to a seedling curtain, and then cultivating till the seedlings are brought out from the storage; and after cultivating the seedlings by strains on the sea, strictly screening according to the original fine variety standard characteristics, and performing selfing cultivation on 10-20 kelps on a raft frame with obvious characteristics. According to the method disclosed by the invention, the problems of mixing and degradation of improved varieties of the kelps produced in the kelp cultivation area in the northern area can be solved, and the method can be used for purifying and optimally selecting the original fine varieties of the kelps.