The invention discloses a compound microorganism silage additive and an application to preparation of grain amaranth silage. The compound microorganism silage additive comprises an enzyme and bacteria compound and an amaranth root extract, enzyme in the enzyme and bacteria compound comprises cellulase, hemicellulose, amylase, pectinase, protease and the like, bacteria in the enzyme and bacteria compound comprise a lactobacilli flora, a bacillus thuringiensis group, photosynthetic bacteria, a yeast flora, an actinomycete group, filamentous bacteria and the like, and main components of the amaranth root extract include plant sterol, polysaccharide, oleanolic acid, flavonoid, triterpenoid saponins, organic acid and the like. Compared with a conventional silage additive, the compound microorganism silage additive is good in fermentation effect and palatability. The grain amaranth silage is prepared from the compound microorganism silage additive, the potential advantage of raw materials serving as green feed can be effectively improved, daily gain can be remarkably increased, feed-meat ratio is increased, animal growth is promoted, and the grain amaranth silage can be widely used for livestock and poultry breeding and aquaculture.