The invention relates to a preparation method of gamma-aminopropyl trisiloxane. The method comprises the following steps: mixing gamma-aminopropyl methyldialkoxylsilane, hexamethyldisilazane and a catalyst according to a ratio, increasing the temperature to reaction temperature, dropwise adding deionized water, keeping the temperature for 6-10 hours after completing dropwise adding, and finally carrying out reduced-pressure distillation to obtain a gamma-aminopropyl trisiloxane product. Compared with the conventional method, the preparation method of gamma-aminopropyl trisiloxane is free of noble metal catalysts; the reaction process is completed in one step; the operation is simpler.