The invention discloses a vacuum ball milling method for improving the compaction density of a lithium iron phosphate material. The vacuum ball milling method comprises the following steps: step a, feeding pre-calcining or sintered lithium iron phosphate into a ball mill through a feeding hole; step b, connecting the ball mill with a vacuum pump through an exhausting nozzle, vacuumizing the ball mill to form ultimate vacuum, filling nitrogen, repeating the procedures for three times, closing the exhausting nozzle, and ensuring the atmospheric pressure to be -0.1 MPa; step c, introducing circulating cooling water into a cooling water jacket, and starting ball milling; step d, after the ball milling, introducing an inert gas, and when the temperature in the ball mill is reduced to be 80 DEG C below, opening a discharge hole to discharge. Therefore, the vacuum ball milling method not only improves the compaction density of lithium iron phosphate, but also can prevent the lithium iron phosphate material from being oxidized.