The invention discloses a method for extracting carrageenan, which comprises the following steps: washing a raw material with water, performing combined alkali treatment on the raw material, wherein the concentration of alkali is decreased gradually, washing the raw material to enable the pH to be 7-8; carrying out composite enzyme treatment, removing heavy metals by using a metal chelating agent, boiling carrageenan, filtering coarsely, filtering finely, reducing temperature, gelling, dewatering, cutting into strips, drying, crushing and screening so as to obtain the carrageenan. According to the invention, a mixed solution of NaOH, KOH and LCI with gradually decreased concentration in three grades is adopted in an alkali treatment process, the loss of carrageenan is reduced effectively while the alkali treatment time is reduced; cellulase and protease combined treatment is adopted, and the yield of the carrageenan is improved while the high gel strength of the carrageenan is ensured; ethylene diamine tetraacetic acid and nopotassium phosphate are adopted for further treatment for the raw material, and the heavy metals in the raw materials are removed effectively. The gel strength of the carrageenan prepared by the method is greater than or equal to 2139.2 g / cm<2>, the extraction rate is greater than or equal to 23.9 percent, Pb is less than or equal to 2.5 mg / kg, As is less than or equal to 1.0 mg / kg, Hg is less than or equal to 0.5 mg / kg, and the Cd is less than or equal to 1.0 mg / kg.