Kelp (Saccharina japonica) is one of the world’s most important aquaculture species and is considered an excellent marine food rich in dietary fiber. In traditionally farmed kelp, the protein content accounts for 6%–9% of dry weight, whereas young kelp from short-cycle cultivation exhibits a uniquely high protein content. These “kelp sprout” possess significant commercial value as a high-protein kelp food source. Research has found that during kelp growth from the seedling stage to the juvenile stage (< 1 m), crude protein content shows a significant downward trend as the cultivation period lengthens. Its content is significantly negatively correlated with thallus length, width, and fresh weight, exhibiting a typical growth dilution effect. Protein content decreases from 23% to 16%–20%. A significant drop in protein content occurs once the average length of kelp reaches 50 m. The optimal harvest window is between 40 and 55 days, during which protein content and yield are in relative equilibrium.We further selected open and semi-open waters in Sansha Bay, China, for aquaculture. By comparing different water areas, species, cultivation batches, and methods, we established a cultivation technique for high-protein “kelp sprout” providing critical technical support for the targeted cultivation of high-protein “kelp sprout” and for improving the quality and efficiency of the industry.