The invention discloses a method for synthesizing a Li4Mn5O12 sub-micrometer rod, belonging to the technical field of synthesis of active material of a lithium ion battery. The method includes the following steps: firstly, synthesizing a MnOOH sub-micrometer rod through hydrothemal method and secondly preparing Li4Mn5O12 through solid phase technique, wherein the MnOOH sub-micrometer rod is synthesized by taking MnSO4.H2O, KMnO4 and CTAB as raw materials through the steps of keeping the materials warm for 12h-30h under 140 DEG C to 180 DEG C, washing through alcohol and deionized water, and drying; and the Li4Mn5O12 is prepared by taking MnOOH and LiOH.H2O as raw materials through the step of keeping the materials warm for 10h-24h under 500 DEG C-900 DEG C. The method has the advantages of easy implementation, good repeatability and low material cost, and the product of Li4Mn5O12 is of one-dimensional structure which facilitates the transfer of the Li +/charges in the one-dimensional direction when the Li4Mn5O12 sub-micrometer rod is applied on the lithium ion battery and is good for improving the performance of the lithium ion battery.