The invention relates to the technical field of trisilylamine preparation, in particular to a method for preparing trisilylamine at an ultralow temperature. According to the method for preparing the trisilylamine at the ultralow temperature without participation of the solvent, by-products generated in the reaction can be easily removed through filtration of a metal ion adsorption permeable membrane, the GC purity of the trisilylamine finally obtained through the reaction reaches 99.5%, the yield reaches 85% or above (based on ammonia gas), and the purity of metal ions contained in the product reaches 6N level; and the device is simple and convenient, the raw material reaction degree is high, the cost is lower, the market value is great, and the industrial production benefit is better.