The invention provides a method for preparing fucoidan, relating to the field of production and application of food additives. The method comprises the following steps of: adding distilled water according to a material to liquid ratio of 1:40-4:80 g / mL with brown algae, such as kelps, undaria pinnatifida, and the like as raw materials; treating by a colloid mill and then carrying out microwave-assisted extraction or ultrasonic wave-assisted extraction, wherein, in the microwave-assisted extraction process, the microwave time lasts 20-40min, the microwave extraction temperature is 60-80 DEG C, and the microwave power is 400-500W; and in the ultrasonic wave-assisted extraction process, the ultrasonic power is 800-100W, the microwave time lasts 20-40min, and the extraction temperature is 60-90 DEG C; and centrifuging an extracting solution, adding a proper amount of 2% CaCl2 solution into a supernatant, stirring, keeping constant temperature for 20-30h at 35-40 DEG C, centrifuging, adding 50-100% ethanol into the supernatant until the ethanol concentration of the solution is 15-20%, stirring, filtering, concentrating, adding 90-100% ethanol into a concentrated solution until the ethanol concentration of the solution is 55-60%, centrifuging again and drying a precipitate in vacuum to obtain white or yellow-white fucoidan powder.