The invention discloses a method for preparing a salt-resistant and detergent-resistant alginate fiber, the fiber being a salt-resistant and detergent-resistant alginate fiber prepared by incorporating a borate-containing adjuvant, contacting and infiltrating a finished alginate fiber with the adjuvant solution and using selected proper raw materials in selected proportions and reaction conditions. The preparation method of the invention solves the problem that the alginate fiber and a fabric made therefrom are neither salt-resistant nor detergent-resistant. The borate-modified alginate fiber and a fabric made therefrom have excellent salt resistances, and can be washed with an alkaline detergent. The swelling degree of the alginate fiber modified by the adjuvant solution can be reduced to 39.8% after the fiber is immersed in a normal saline at 30°C for 72 h and can be reduced to 55.3% after the fiber is immersed in a standard detergent for 24 h, while the fiber maintains its original morphology without any obvious dissolution phenomenon, and the self flame-resisting performance of the alginate fiber is also maintained.