The invention discloses a method for preparing a magnesium oxysulfate short fiber material by using a magnesium process flue gas desulfurization product magnesium sulfite one-step process, and belongsto the technical field of inorganic nonmetallic materials. According to the method, magnesium sulfite and magnesium oxide are placed in a high-pressure reactor at a certain mass ratio, a hydrothermalreaction is conducted under the condition that oxygen is continuously introduced, and the magnesium oxysulfate short fiber material can be directly prepared. According to the method, the magnesium sulfite raw material is a magnesium sulfite by-product generated in the magnesium process flue gas desulfurization process, the clean and recycling utilization of the magnesium process flue gas desulfurization by-product magnesium sulfite is achieved, the magnesium oxysulfate short fiber material with the excellent performance can also be obtained, the magnesium oxysulfate short fiber material has the characteristics that dispersing is easy, the length is even, and the size is fine and small; and the production process is short, the cost is low, the raw material is easy to obtain, discharging ofthree wastes is avoided, and the great significance is achieved for clean production of a magnesium process flue gas desulfurization technology in China.