The invention relates to a method of manufacturing triisopropanolamine. Firstly, air in a reaction kettle is replaced by nitrogen, and the reaction kettle is vacuumized; then after being injected with the nitrogen, the reaction kettle is vacuumized; the step is repeated for three to five times; secondly, isopropanolamine or diisopropanolamine or isopropanolamine mixture are put in the reaction kettle, the nitrogen is injected until the pressure reach more than minus 0.05MPa, then propylene oxide is added in continuously, wherein, the molar ratio between the propylene oxide and the number of hydrogen atoms contained on nitrogen atoms in the isopropanolamine is controlled within 1.0 to 1.1, the reaction temperature ranges from 50 to 90 DEG C, the reaction pressure ranges from minus 0.1 to 1.0MPa, and the reaction time is 8 to 15 hours; after the reaction is finished, products are discharged and packed under the protection of the nitrogen. The method greatly lowers the energy consumption, reduces the caused environmental pollution, increases the product quality, has low demand to equipment, saves the cost, has wide demanding range to raw material, and can make the best of waste materials which are generated when manufacturing the isopropanolamine and the diisopropanolamine. The purity of the products which is manufactured by adopting the method can reach more than 99 percent, the moisture content is less than 0.3 percent, and the colority of Pt-Co is less than 50.