The invention provides a synthetic method of N-tert-butyl diethanolamine. The synthetic method comprises the following steps of: a, putting a catalyst into a reactor, replacing the air in the reactor by using nitrogen, vacuumizing the reactor until the pressure is between -0.1MPa and -0.03MPa, adding metered tert-butylamine, then raising the temperature in the reactor to be 50-160 DEG C, slowly adding metered epoxyethane to ensure that the temperature in the reactor is kept between 50 DEG C and 160 DEG C, and keeping the pressure to be 0-0.8MPa until the reaction is finished to obtain a mixture containing N-tert-butyl diethanolamine; b, performing post treatment; and c, purifying to obtain the N-tert-butyl diethanolamine. Compared with the prior art, the synthetic method adopts a charging mode with negative pressure in the reactor and an appropriate catalyst, the raw material partial pressure is improved, the reaction speed can be further increased, oxidation discoloration of a product can be prevented, the reaction condition is relatively mild, the operation is more controllable, the yield is high, the product quality is more ensured, the operation is simple and the cost is low, so that the synthetic method is favorable for industrial production.