NOVELTY - Extraction of active ingredient from Enteromorpha comprises using technology A, which includes mixing Enteromorpha powder with water, adjusting pH of the mixture to 1-2 by acid, heating the mixture at 50-60 degrees C, and extracting for 3-4 hours; or technology B, which includes mixing Enteromorpha powder with water, adjusting pH of mixture to 9-10 by alkali, heating the mixture at 85-95 degrees C, and extracting for 4-5 hours.    USE - The method is for extracting active ingredient from Enteromorpha useful for preparing Enteromorpha seaweed fertilizer, i.e. foliage fertilizer, flushing fertilizer, or granular fertilizer (claimed), for use in enhancing photosynthesis of crop, increasing output of crop, improving quality of product, resisting cold, drought, degeneration, and plant disease, and improving the ratio of bearing flower and fruit of crop. The Enteromorpha seaweed fertilizer is useful for modern agriculture.    ADVANTAGE - The method provides Enteromorpha seaweed refined powder with high activity. It provides Enteromorpha seaweed fertilizer, which has low production cost and good solubility and is safe to use. The seaweed fertilizer is natural and environment friendly; and provides no harmful effect to people or domestic animals.    DETAILED DESCRIPTION - Extraction of active ingredient from Enteromorpha comprises: (A) using fresh Enteromorpha as raw material, removing sand in Enteromorpha, squeezing and dehydrating, and drying and grinding; (B) extracting active ingredient of Enteromorpha by technology A or technology B, where technology A comprises mixing Enteromorpha powder with water, adjusting pH of the mixture to 1-2 by acid, heating the mixture at 50-60 degrees C, and extracting for 3-4 hours; and technology B comprises mixing Enteromorpha powder with water, adjusting pH of mixture to 9-10 by alkali, heating the mixture at 85-95 degrees C, and extracting for 4-5 hours; (C) separating and removing slag of Enteromorpha; adjusting pH of supernatant liquid to 7-8 by alkali or acid; and (D) condensing the supernatant liquid in vacuum until its specific gravity is 1.15-1.25 g/ml. An INDEPENDENT CLAIM is included for a method of preparing Enteromorpha seaweed fertilizer comprising using extraction liquid of Enteromorpha as basic liquid and combining the basic liquid with humic acid, carbamide, monopotassium phosphate, potassium sulfate, monoammonium phosphate, and trace element.