The invention provides N-octadecyl methyl ethylamine and a synthesis method thereof. The synthesis method comprises the following steps: dissolving acetal and octadecylamine into methanol, wherein themolar ratio of the acetal to the octadecylamine ranges from1:1 to 1.5:1, and the weight of the methanol is 50 to 99 percent of the total weight; heating to appointed temperature and performing normal-pressure reaction for 0.1 to 12 hours; reducing the pressure to 0.01 to 0.08 MPa, carrying out a reaction at 20 to 120 DEG C for 0.25 to 12 hours, and stopping the reaction to obtain the N-octadecylmethyl ethylamine. The yield of the N-octadecyl methyl ethylamine synthesized by the synthesis method of the N-octadecyl methyl ethylamine can reach up to 71 percent or higher, the synthesis method issimple in operation, the volatile products are easy to recover, and environmental friendliness is achieved.