The invention discloses a preparing method of seaweed soy sauce, comprising the steps of 1. washing seaweed cleanly and crushing, adding the crushed seaweed into a steam cooker, adding water and citric acid solution, boiling, extracting for 1.5h under the condition of 85-90 DEG C, separating centrifugally, filtering, adding sodium carbonate into the filtrate, and neutralizing, sterilizing, and thereby obtaining seaweed stock solution; 2. preparing soy sauce according to traditional steps, adding the seaweed stock solution during the leaching process, blending and sterilizing, thereby obtaining the finished soy sauce. According to the preparing method of seaweed soy sauce, provided by the invention, bean cake, bran and seaweed are used as principal raw material and are fermented to be processed into the soy sauce with seaweed flavor, and thus, the seaweed soy sauce not only keeps nutritional ingredients of soy sauce, but also maintains health-care ingredients of seaweed, and is a good source for supplying iodine for people.