The invention discloses biomass pellet feed for ruminants and a preparation method thereof. The biomass pellet feed comprises the following components in parts by weight : 32-40 parts of corn stalks,18-24 parts of corn cobs, 27-33 parts of sweet potato stalks, 16-26 parts of ryegrass, 21-25 parts of brewer's spent grains, 11-15 parts of eggshells, 20-24 parts of soybean meal, 0.7-1.3 parts of preservatives, 2-4 parts of antioxidants and 0.6-1.2 parts of choline chloride. Te preparation method includes the following steps: Step 1, using ammonia water, calcium hydroxide solution and carboxamideperoxide to carry out combined chemical impregnation on the corn stalks, the corn cobs, the sweet potato stalks and other biomass materials, and carrying out saturated steam explosion treatment at the pressure of 2.4-3.6 MPa, thereby obtaining a pretreated biomass material; Step 2, subjecting the pretreated biomass material to dehydration drying and ensiling fermentation, thereby obtaining silage; Step 3, evenly mixing crushed egg shells with the preservatives, the antioxidants and the choline chloride, thereby obtaining a feed additive; and Step 4, evenly mixing the silage with the feed additive, and subjecting the mixture to granulation and magnetic sterilization, thereby obtaining the biomass pellet feed. The invention can further increase the softness and digestibility of the biomasspellet feed and accelerate the ensiling fermentation rate by the combined chemical impregnation and steam explosion of cellulose molecular chains, chemical bonds, hydrogen bonds and crystallinity; andthe biomass pellet feed is enhanced in the preservative performance and prolonged in the storage time by cooperating the choline chloride with the preservatives.