The present invention provides methods for preparing trimanganese tetroxide with low BET specific surface area and methods for controlling particle size of trimanganese tetroxide and trimanganese tetroxide product. The preparation methods and the methods for controlling particle size comprise following processes: (1) Purifying air by spraying dilute aqueous ammonia; (2) Pretreating of removing impurities from MnSO 4 solution with concentration in a range of 150~200g/L; (3) Process for preparing seed crystal: Cooling MnSO 4 solution to a temperature of less than 40°C, and introducing liquid NH 3 into the cooled MnSO 4 solution until pH value reaches 10.5-11.0, and then separating the mixture by solid-liquid separation to obtain filtrate and solid; adding NH 4 HCO 3 to the filtrate to recovery Mn element, and then forming slurry, introducing the purified air, and oxidizing the slurry into Mn 3 O 4 ; (4) Process for obtaining the final product by controlling oxidization: adding Mn 3 O 4 seed crystal to the MnSO 4 solution, Al containing salt is optionally added, turning on the recycle pump, controlling the temperature of the system in the range of 25 ± 5°C, and controlling pH value of the reaction system in a range of 6.5-7.5, which makes the MnSO 4 solution circulate more than 6 times per hour, not stopping the reaction until MnSO 4 content of the reaction solution is ≤1.5g/L, so as to obtain Mn 3 O 4 product.The trimanganese tetroxide product has properties that the particle is spherical, impurities content is low, the crystalline phase is pure, particle size distribution is narrow, which meets the international requirements for trimanganese tetroxide used for preparation manganese cathode materials for lithium ion secondary battery. The trimanganese tetroxide product has properties that BET specific surface area is less than 1m 2 /g, D 50 is 10-12µm or 14-16µm, weight content of Fe is less than 10ppm, and weight content of Cu, Zn, Pb and Cd are less than 0.1ppm.