The invention belonging to the field of cosmetics, and particularly relates to a Sargassum horneri boiled alga clay mask and a preparation method thereof. The preparation method comprises the following steps: removing impurities from Sargassum horneri, washing, drying until the algae become embrittled, stamping the dried Sargassum horneri to granules, immersing in alcohol for 8-20 hours, filtering, boiling for 15-120 minutes, filtering to obtain alga clay, carrying out centrifugal dehydration and freeze-drying, and grinding into powder; weighing raw materials according to the formula; adding deionized water into the weighed Sargassum horneri powder, and stirring to swell; and dissolving a moisturizer, an antioxidant, a metallic ion chelator, a preservative, a whitener, a deodorizer, a sebum inhibitor, a cutin dissolving agent, a bacteriostatic agent, an antiallergic agent and a nutrient additive in deionized water, adding the Sargassum horneri clay, continuing stirring while adding a tackifying agent and deionized water, and stirring uniformly. The mask has the advantages of favorable moisturizing property, low cost and low price, obviously improves the skin color, moisturizes the skin, and prevents the skin from aging.