The present invention discloses a method for directly synthesizing sodium borohydride by solid-state ball milling at room temperature, which comprises: performing solid-state ball milling on a mixture of a reducing agent and a reduced material by using a ball mill under room temperature, and performing purification to obtain sodium borohydride. The reducing agent comprises one or more of magnesium, magnesium hydride, aluminum, calcium, and magnesium silicide. The reduced material is sodium metaborate containing crystallization water or sodium metaborate, or is a mixture of sodium metaborate containing crystallization water and sodium metaborate. The solid-state milling is performed in a mixed atmosphere of argon and hydrogen, or an argon atmosphere, or a hydrogen atmosphere. The present invention has a simple process, a controllable and adjustable reaction procedure, mild reaction conditions, low energy consumption, low costs, high yield, no pollution, good safety, and easy industrial production.