The invention relates to a method for preparing seaweed fiber. The method comprises: adding pretreated seaweed raw material to digestive juice for digestion; filtering digested solution; taking filtrate as spinning solution to spin; and adding cross-linking agents in the digestion or spinning process. The method directly takes the solution obtained from the seaweed raw material through treatment,digestion and filtration to spin. Compared with conventional seaweed fiber spinning process routes, the process route reduces dilution, washing, calcium precipitation, decalcification, precipitation,filtration, drying, grinding, sodium alginate formation, sodium alginate dissolution, defoaming and other processes, greatly reduces cost, and also avoids using chemical reagents causing serious pollution.