Salt and detergent resistance has been a problem in preparation of alginate fiber. In order to solve this problem, the alginate fiber was modified with sodium tetraborate. The modified alginate fiber was characterized by FT-IR and XRD. The liquid absorption, calcium ion content, breaking strength, micromorphology of fiber and spinning solution were tested to study the salt and detergent resistance of modified alginate fibers. The results show that the modification of alginate fibers with sodium tetraborate can increase the degree of cross-linking and the crystallinity of seaweed fibers, thereby reducing the damage of calcium and sodium ion exchange in saline and detergent to the structure of seaweed fibers, which is beneficial to maintain the fiber morphology. After soaking in saline and detergent, the liquid absorption of the alginate fiber is only 10.2% and 10.1%, and the breaking strength loss is only 5.1% and 9.0%, which significantly improves the salt and detergent resistance of alginate fibers.