A method for manufacturing a fucoidan cosmetic composition includes: washing mozuku to eliminate residual salt and impurities; adding citric acid to the washed mozuku and pulverizing the mozuku; adding water to the pulverized mozuku and performing reflux extraction to obtain a high molecular weight fucoidan; adding a neutralizing agent to the high molecular weight fucoidan for neutralization; adding diatomite to the neutralized high molecular weight fucoidan and causing a reaction for 30 minutes for deodorization; filtering the deodorized high molecular weight fucoidan to eliminate insoluble substances; adding a lactic acid bacterium to the high molecular weight fucoidan removed of the insoluble substances and causing fermentation to obtain a low molecular weight fucoidan; concentrating the purified low molecular weight fucoidan with a reduced pressure evaporator; and powdering the concentrated low molecular weight fucoidan through freeze drying.