The invention discloses an instant kelp cake and a preparation method thereof. The kelp cake comprises the following components in parts by weight: 30-50 parts of kelp pulp, 20-30 parts of starch, 0.05-0.2 part of konjac gum, 3-6 parts of vegetable oil, 4-10 parts of white granulated sugar, 1-5 parts of maltose syrup, 1-3 parts of citric acid, 1-3 parts of malic acid and 0.2-1 part of mannitol. The preparation method of the kelp cake is as follows: subjecting dried kelp to rehydration, putting the rehydrated kelp into a fishy smell-removal agent solution for removing fishy smell, conducting softening and pulping to obtain kelp pulp, then adding auxiliary materials, and conducting blending, steaming, cooling, molding, sterilization and packaging to obtain the kelp cake. The invention has the beneficial effects that the kelp cake is good in appearance, color, texture and mouth feel, high in acceptance degree and free from kelp fishy smell; the preparation method can completely remove thekelp fishy smell, maintain the nutrient components of the kelp without loss, reduce the hardness of the kelp, and improve the sensory acceptance of the kelp cake; and the fishy smell-removal agent can be reused, and is low in cost.