The invention provides a slag ball content control method for preparation of mineral wool from silicon-manganese slag and relates to improvement of a technological method for preparing mineral wool from metallurgical slag, particularly from the silicon-manganese slag. The method is characterized in that a flux is added to hot melt slag of a silicon-manganese alloy in the preparation process, wherein the flux contains calcium fluoride and magnesium oxide, adding amount of calcium fluoride is 5%-8% by weight of the hot melt slag of the silicon-manganese alloy, and the adding amount of magnesiumoxide is 3%-5% by weight of the hot melt slag of the silicon-manganese alloy. According to the method, the slag is diluted by adding the flux to the hot melt slag of the silicon-manganese alloy, flowability of the hot melt slag is enhanced, fibers can become thinner due to wool forming in a high-speed centrifugation state, content of slag balls in the mineral wool is reduced, tensile strength of the mineral wool is improved, cost is lower, and operation is facilitated; glass state of high-temperature melt of the prepared mineral wool is good, fiber forming rate is high, fiber forming quality is excellent, slag ball content is lower than 8%, heat conductivity coefficient is low, tensile strength is high, and diversified market demands can be met.