The invention relates to kelp sauce and a preparation method thereof. The preparation method comprises the following steps: firstly cleaning and soaking kelp, adding 15 to 25 percent of edible salt and 200 to 400 percent of water of the weight of the kelp to the kelp, placing the kelp in a heater for heating, when heating to between 40 and 45 DEG C, adding 2 to 10 percent of edible alkali of the weight of the kelp, continuously heating until the kelp becomes pasty, cooling the kelp to below 30 DEG C, and grinding the kelp into kelp paste; after mixing the kelp paste with flour paste, garlic sauce, chilli sauce, salt, sugar and vinegar evenly, heating the mixture to prepare kelp sauce, bottling the kelp sauce after the kelp sauce is cooled, and then exhausting, sealing and sterilizing the kelp sauce, wherein the kelp sauce comprises the following components in weight percentage: 50 to 70 percent of the kelp paste, 10 to 20 percent of the flour paste,1 to 3 percent of the garlic sauce, 5 to 12 percent of the chilli sauce, 1to 5 percent of the salt, 5 to 12 percent of the sugar and 1 to 3 percent of the vinegar. The kelp sauce has the advantages of reddish brown color, delicious hot flavor, enough garlic fragrance, fresh taste and rich nutrition. The preparation method has the advantages of reasonable preparing technique, simple and convenient preparation, and strong operability, and is easy to achieve large-scale production.