The invention discloses a method for extracting phycocyanin from phormidium tenue, which comprises preparing sodium nitrate, dibasic potassium phosphate, magnesium sulfate, calcium chloride, citrate, ammonium iron citrate, disodium editate, sodium carbonate and micro element solution into phormidium tenue culture medium, arranging phormidium tenue into the culture medium to cultivate and prepare phormidium tenue powder, extracting and purifying phormidium tenue phycocyanin from the phormidium tenue powder. The invention has simple process, easy operation, high reliability, high yield, high quality, simple device and application for industrial production.