The invention provides a preparation method of 3-methylthio propylamine. The method is characterized in that under the effects of microwaves and ultrasonic waves, flavoring essence of 3-methylthio propyl alcohol capable of being easily obtained in China is used as a starting raw material; the reaction time is short; the total yield is relatively high. Under the effects of microwaves and ultrasonicwaves, firstly, 3-methylthio propyl alcohol and thionyl chloride react for 20 to 60 min under the back flow condition to prepare 3-methylthio-1-chloropropane, and the yield is greater than 95 percent; then, at 80 to 90 DEG C, the 3-methylthio-1-chloropropane and phthalimide potassium react for 0.5 to 2.5h to prepare N-3-methylthio propyl phthalimide, and the yield is greater than 94 percent; finally, the N-3-methylthio propyl phthalimide and hydrazine hydrate perform backflow reaction in absolute ethyl alcohol for 20 to 60min to prepare the 3-methylthio propylamine, and the yield is greater than 85 percent.