The invention discloses a kelp beverage for lowering blood pressure and a preparation method thereof. The preparation method comprises the following steps: (1) crushing dried kelp, mixing the crusheddried kelp with clean water, heating and stirring the mixture, and draining the mixture to collect first leachate; (2) adding clean water, sodium carbonate, disodium hydrogen phosphate into a solid material obtained in step 1, uniformly stirring the mixture, carrying out ultrasonic treatment, and draining the mixture to collect second leachate; (3) merging the first leachate and the second leachate, introducing nitrogen, and keeping stirring; (4) inoculating kluyveromyces marxianus and streptococcus thermophilus into mixed liquid obtained in step 3, and carrying out fermentation and heat inactivation; and (5) filtering fermentation broth to obtain the kelp beverage. The kelp beverage for lowering blood pressure has characteristics of good mouthfeel, mild property to intestines and stomach,relatively light color and luster, etc., has a prospect of being further prepared into other beverages, and provides a good soft beverage for people who are not suitable for drinking alcoholic beverages, such as hypertensive patients, etc.