The invention discloses kelp yoghurt with a function of relaxing the bowel and a preparation method of the kelp yoghurt. The kelp yoghurt is prepared from raw materials in parts by weight as follows:50-60 parts of kelp, 20-36 parts of aloe, 15-18 parts of fructus crataegi, 10-12 parts of mulberries, 25-30 parts of purple sweet potato powder, 10-15 parts of stachyose, 20-25 parts of spirulina powder, 25-30 parts of honey, 15-20 parts of soybean dietary fiber, 100-120 parts of fresh milk, 20-25 parts of dried skim milk, 1-3 parts of a compound stabilizer and 1-5 parts of microorganism inoculant. The preparation method comprises steps as follows: 1) treatment of kelp; 2), blending, homogenizing and degassing; 3) sterilization; 4) cooling, inoculation and fermentation; 5) cooling, stirring and canning; 6) post-maturation. The kelp yoghurt is rich in nutrition and convenient to carry and has effects of alleviating mental depression, harmonizing the stomach, strengthening the spleen, regulating qi and adjusting intestinal microenvironment, the raw materials are scientifically compatible, and the function of relaxing the bowel is remarkable.