Silage, as a traditional method for preserving animal feed, can be a major source of energy for animals throughout the year. It is widely used in commercial livestock production in most developed countries. High-quality silage is an important foundation for the development of animal husbandry. The main fermentation mechanism of silage is that fresh forage crops and grasses are cut, chopped, and sealed in silos, forming a relatively anaerobic condition for effective preservation. The lactic acid bacteria use the water-soluble carbohydrates in the raw materials for fermentation, producing lactic acid, causing a decrease in pH value and inhibiting the growth of aerobic bacteria, effectively preserving high nutrient content and improving feed palatability. Moisture, water-soluble carbohydrates, lactic acid buffering capacity, and epiphytic microorganisms in forage crops and grasses are the dominant factors affecting silage fermentation, and this chapter focuses on the principle of interaction of these factors.