The invention relates to a biological organic fertilizer, and particularly relates to a biological organic fertilizer prepared by using mushroom bran as a raw material and a preparation method thereof. The biological organic fertilizer is prepared by mixing mushroom bran, humic acid, raw chicken manure and a composite bacterial agent according to weight parts, and performing fermentation. On a weight basis, the ratio of the mushroom bran, humic acid, raw chicken manure and the composite bacterial agent is 1:0.08-0.1:0.08-0.1:1/300-1/400. The fertilizer of the invention can eliminate biological pollution of the mushroom bran; nutrient substances of the mushroom bran are made full use of for the production of the fertilizer; no harm is caused to the environment; and the reutilization of resources is realized.