The present invention belongs to the field of nutritional food and particularly relates to a fully nutritional cereal bar and a preparation method thereof. The cereal bar is prepared from the following raw materials: puffed chenopodium quinoa, oatmeal, black rice and red date puffed rice, a chenopodium quinoa peptide extract, hazelnut kernels, black sesame seeds, pumpkin seed kernels, dried red dates, water-soluble dietary fibers, soybean peptides, L-glutamine, glucose syrup, high oleic acid sunflower seed oil, glycerol, soybean phospholipids, soybean protein isolate, and a compound nutrition enhancer. The provided cereal bar is prepared from the chenopodium quinoa as the raw material, combines other cereals, is balanced and comprehensive in nutrition, and has effects of improving an anti-oxidizing ability and immunity, and prolonging a shelf life by adding the chenopodium quinoa peptide extract. After a nutrition enrichment, the product has high quality proteins accounting for 50% of the total proteins, is increased in contents of various vitamins and minerals, and meets requirements of whole nutrition food.