The invention discloses a method for preparing a liquid seaweed fertilizer. The seaweeds serve as raw materials, and the method sequentially comprises the following steps: pulping and crushing fresh seaweeds or soaking and swelling dried seaweeds for pulping and crushing, performing ultrasonic extraction, performing solid-liquid separation, performing alkaline extraction on solid seaweeds, compounding the seaweed extraction solution and common fertilizers, thereby obtaining the liquid seaweed fertilizer. According to the method disclosed by the invention, the extraction efficiency of active ingredients in the seaweeds can be obviously improved, and compared with a traditional chemical extraction method, the method disclosed by the invention has the advantages that because a mild method is adopted, for example, ultrasonic extraction and relatively low temperature and dilute acid/dilute alkali assisted ultrasonic extraction are adopted, and the alkali extraction time is obviously shortened. According to the process technology, the activity of the active ingredient can be obviously preserved in the extraction process, so that high-activity seaweed extract and products thereof are obtained.