A method for preparing lithium borohydride by means of room temperature solid phase ball milling, comprising the following steps: uniformly mixing a magnesium-containing reducing agent and a lithium metaborate-containing reducing material under a non-oxidizing atmosphere at room temperature, performing solid phase ball milling, isolating and purifying to obtain lithium borohydride. The method has the advantages of having a simple process, having a controllable and adjustable reaction procedure, having mild reaction conditions, energy consumption being low, costs being low, and output being high, while creating no pollution, being safe and cyclically using boron resources, having important practical significance.