The invention provides a synthetic method of trisilylamine. The preparation method comprises the following steps: using inert gas for purging a catalytic distillation tower with inert gas in advance,respectively, simultaneously and continuously adding chlorosilane and ammonia in the catalytic distillation tower from a rectifying section and a stripping section in a gaseous state for a reaction, wherein the mol ratio of chlorosilane to ammonia is 1:1.2-3, the space velocity of chlorosilane is 0.5-2 / h, the reaction pressure is 0.2-1 MPa, and the reaction temperature of the distillation tower is350-420 DEG C, continuously leading the reaction products in the tower bottom out, separating the material to obtain NH4Cl, and using a widely-known impurity-removal refining technology to obtain thehigh purity trisilylamine.