To investigate extracting technique and potential biological activity of fucoidin in Undaria pinnatifid, a kind of marine organisms. Factors such as solid-liquid ratio (times), time of extraction (min) and temperature of extraction (°C) were chosen to be investigated to examine the best extracting technique. 50 mice were randomed to negative comparison group, positive comparison group and experiment groups. Mice were inoculated tumor stock S180. The negative comparison group were administrated with physiological saline, positive comparison group administrated with astragalus polyose(50mg/(kg•d)), and experiment groups administrated with fucoidin(100mg/(kg•d), 50mg/(kg•d), 25mg/(kg•d)). The tumors were peeled and weighted after 7 days. The best extracting condition is definite to be 100 °C, 60 times of water and 7h. The content of fucoidin in Undaria pinnatifida is 55.4％ after preliminary purification. The tumor inhibition rate of experiment groups are 56.19%, 65.71% and 30.48%. The results of investigation on orthogonal experiments are reliable. And fucoidin in Undaria pinnatifida has certain antitumor effect.