The invention relates to a seaweed biological fiber and a preparation method and application thereof. The seaweed biological fiber comprises, by weight percentage, 0.1-25.0% of microalgae and 99.9-75.0% of seaweed biological macromolecule. According to the method for preparing the seaweed biological fiber, the seaweed biological macromolecule is dissolved in microalgae water solution in proportion to form uniform spinning solution, solid liquid is squeezed into the spinning solution through spinneret holes to form fiber, and the seaweed biological fiber is obtained after undergoing washing and drying. Microalgae plants having good biological activity are added into blend fiber, so that the seaweed biological fiber contains various bioactive substances good for human body, and the skin friendliness and the health care function of the fiber and spinning materials processed by the fiber are increased. The seaweed biological fiber is especially applicable to various medical and health spinning materials in close contact with human body, such as facial masks, cosmetic, medical dressings, baby diapers, women sanitary articles and adult incontinence articles and so on.