A method of preparing a brown seaweed extract containing alginate is disclosed. The method includes washing brown seaweed containing alginate to remove impurities, mixing alginate contained in the brown seaweed with glycerin by precipitating and soaking at a temperature of 70 to 80° C. for 4 to 6 hours after precipitating the brown seaweed, where the impurities are removed, in 110-120 parts by weight of a mixture including water and glycerin based on 100 parts by weight of the brown seaweed soak, stirring and maturing the brown seaweed precipitated and soaked in the mixture at a temperature of 5° C.-40° C. for three to seven days, and eluting by compressing the matured brown seaweed with the mixture, and concentrating the brown seaweed extract by distilling water at a temperature of 75 to 105° C. for 4 to 6 hours after adding the eluted extract to a vacuum condenser.
