The invention discloses biological organic fertilizer and a preparation method thereof. The biological organic fertilizer is prepared from the following raw materials: animal feces, sawdust, zymophyte, urea, trace elements, an antiviral agent, plant ash and vinasse. The biological organic fertilizer has the beneficial effects that the biological organic fertilizer takes the animal feces and the sawdust as the raw materials, waste utilization is realized, and environmental pollution is reduced; the biological organic fertilizer can provide rich nutritional ingredients for crops, growth of thecrops is promoted, and plant diseases and insect pests can be reduced; and the biological organic fertilizer has a certain improving effect on soil, and has certain purification and repair effects onthe soil. In the production process of the biological organic fertilizer, fermentation are conducted twice, areas for two times of fermentation communicate through a conveying belt, fermentation initial materials obtained by the first time of fermentation are conveyed to the second-time fermentation area through the conveying belt, the contact area of the fermentation initial materials and air isincreased, full contact of the fermentation initial materials and the air is facilitated, and thus the fermentation materials are fermented more perfectly.