The invention belongs to the field of foods, and particularly relates to premixed coarse grain powder and a preparation method thereof. The premixed coarse grain powder is prepared by mixing the raw materials of mixed powder of cereals, Chinese wolfberry fruits, walnuts and sesame seeds, fermented bean powder, Chinese yam powder and the like, wherein the mixed powder of cereals, Chinese wolfberry fruits, walnuts and sesame seeds is obtained through the following steps of mixing black rice with oat and buckwheat, performing soaking, performing inoculation with saccharomyces cerevisiae, aspergillus oryzae and cordyceps sinensis in the fungus count ratio of the saccharomyces cerevisiae to the aspergillus oryzae and to the cordyceps sinensis being (0.5-1.5) to (0.5-1) to (1-3), performing mixed fermentation to obtain cereal type coarse cereal powder, then mixing the cereal type coarse cereal powder with the Chinese wolfberry fruits, the walnuts and the sesame seeds, and performing low-temperature crushing; and the fermented bean powder is obtained through the following steps of soaking red beans, black beans and peas, performing sterilization, performing inoculation with rhizopus, mucor and bacillus subtilis in the fungus count ratio of the rhizopus to the mucor and to the bacillus subtilis being (1-5) to (1-3 ) to (1-3), performing mixed fermentation, and performing crushing. The premixed coarse grain powder provided by the invention is easy to digest, good in dispersion stability and simple in preparation process.