The invention relates to a ruminant feed bioenhancer and a preparation method thereof, which comprises the following steps: a) preparing a fermentation medium, by weight: a carbon source of 85%-95%, a nitrogen source of 1%-2%, The growth promoter is 1%-3%, and the balance is water. The above-mentioned ingredients are crushed, mixed, and set aside; b) inoculating Candida utilis in the fermentation medium obtained in step a), and solid fermentation culture; c) in step a) b) Lactobacillus plantarum is inoculated in the obtained fermentation medium, and anaerobic solid fermentation is cultivated, and OD600 is detected by sampling every 4 hours; d) when OD600 enters the logarithmic growth phase, Bacillus subtilis is inoculated in the fermentation medium obtained in step c), and solid fermentation is carried out. nourish. The biological feed prepared by the method can improve the rumen micro-ecological balance of ruminants, improve feed conversion rate and body immunity, and the product has a special sour-sweet smell, good palatability, and easy for animals to eat.