The invention discloses a method for preparing 3-aminopropionitrile under a supercritical condition. According to the method, liquid ammonia and acrylonitrile are used as raw materials which are subjected to direct addition reaction in a pipeline reactor without adding a catalyst and a solvent to obtain 3-aminopropionitrile. The reaction according to the invention is carried out under a supercritical condition of the liquid ammonia, thus avoiding the use of a catalysts and a solvent when the liquid ammonia is used in reports in the past; a product can be obtained through continuous rectification; unreacted raw materials can be directly used; the process is safe and environmentally friendly, substances of non-reactive raw materials are not introduced, and the method accords with a concept of green chemistry. The conversion rate of acrylonitrile can reach 99.5-100.0%, and the selectivity can reach 85.5-95.8%, so that the acrylonitrile has a high industrial application value.