The invention provides a method for cultivating undaria pinnatifida and using the undaria pinnatifida to treat red tide. The method is characterized in that according to a sign before the occurrence of the red tides, that is, the temperature is increased by 2 DEG C within 7 days, a method different from a biological restraint method is adopted; before the outbreak of the red tides, the cultivatedundaria pinnatifida is added into the sea to absorb nutritional components in the sea water to alleviate the eutrophication of the sea water, and thus the occurrence of the red tides is avoided. The method includes: performing induced cultivation on female gametophyte to obtain young sporophyte, and carrying out aerated feeding cultivation to obtain mature sporophyte; releasing the sporophyll of the sporophyte into the sea to obtain zoospores, and placing seedling curtains attached with the zoospores in fresh sea water; when the germinated zoospores develop into female gametophyte, co-cultivating male gametophyte and the female gametophyte attached to the seedling curtains to complete the development and fertilization process; transferring the young sporophyte to a relevant sea area for cultivation after the young sporophyte reaches 50-100 micrometers to obtain the undaria pinnatifida and treat the red tides.