The present disclosure relates to the technical field of preparation of trisilylamine (TSA), in particular to a method for preparing TSA at an ultra-low temperature. The present disclosure provides a method for preparing TSA without a solvent at an ultra-low temperature, where by-products generated by a reaction can be easily removed by filtration with a metal ion-adsorption permeable membrane. The TSA obtained by the reaction has a gas chromatography (GC) purity of 99.5%, a receivable yield of not less than 85% (in terms of ammonia), and a metal ion purity of 6N. In addition, the method has a simple device, a high reaction degree of raw materials, a lower cost, a great market value, and a better industrial production benefit.