The invention relates to an inulin high-dietary-fiber nutritional cake with the function of preventing and improving constipation and a preparation method of the inulin high-dietary-fiber nutritional cake. The inulin high-dietary-fiber nutritional cake is prepared from the following raw materials in parts by weight: 42 to 48 parts of wheat flour, 10 to 14 parts of maltitol, 12 to 16 parts of palm oil, 7 to 14 parts of inulin, 3 to 6 parts of roasted wheat bran, 2 to 4 parts of oatmeal, 1 to 3 parts of buckwheat flour, 0.8 to 2.0 parts of swelling agent, 0.05 to 0.15 part of milk flavor, 0.1 to 0.2 part of phospholipid, 4 to 6 parts of water and 3 to 4 parts of butter. Wherein the weight ratio of the inulin to the wheat bran is (2-2.5): 1. According to the prepared nutritional cake, the weight ratio of the inulin to the wheat bran is (2-2.5): 1, so that the nutritional cake has the characteristics of being uniform in color and luster, glossy, crisp in taste, not sticky to teeth, clear in gradation, not prone to deterioration and the like, and meanwhile, the nutritional cake reaches the health standards of dietary fibers, low sugar, coarse grains and nutrition.