The invention discloses a preparation method of tetrakaidecahedron-shaped nano lithium manganate. The method comprises the following steps: preparing a lithium manganate seed crystal through a microwave heating method; preparing the nano-grade lithium manganate through a hydrothermal method. According to the method disclosed by the invention, the seed crystal obtained by utilizing a rapid heating effect of microwaves is fine and uniform, and is used as a crystal growth basic point of a subsequent hydrothermal step, so that a product with a small grain diameter and a uniform size is easy to obtain; in a hydrothermal process, amino acid is used as a precipitating agent and a soft template agent; the lithium manganate with a tetrakaidecahedron structure is obtained by utilizing a slow releasing effect and a special steric effect. The tetrakaidecahedron-shaped nano lithium manganate obtained by the invention is used as a positive electrode material of a lithium ion battery; the power density and the battery specific capacity are improved through influences of a special shape on ion diffusion and influences on particle accumulation; the tetrakaidecahedron-shaped nano lithium manganate has a wide application prospect.