A method for decolorizing and deproteinizing brown algae polysaccharides belongs to the field of deep processing of brown algae. The method combines ultraviolet (UV) with hydrogen peroxide (H2O2), including the following steps: extracting dried and pulverized brown algae by hot water to obtain brown algae polysaccharide, dissolving the brown algae polysaccharide in an aqueous solution containing H2O2 and irradiating under ultraviolet light, wherein the mass concentration of the brown algae polysaccharide is 2.5-10.0 mg/mL, the concentration of the H2O2 is 25-150 mmol/L, and the UV irradiation time is 1.0-2.0 h, so as to deproteinize and decolorize the brown algae. The invention does not use acids, bases and organic solvents, which is green with no pollution, simple in operation, safe, economical and time-saving.