The invention belonging to the field of cosmetics, and particularly relates to a Sargassum horneri alga powder 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, and pulverizing the dried Sargassum horneri to granules; regrinding the Sargassum horneri granules into powder, and screening to obtain Sargassum horneri 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, prevents the skin from aging, reduces the wrinkles and is convenient to use.