A preparation method for porous micrometer cubic blocky lithium manganate of a cathode material for a lithium ion battery belongs to the technical field of a chemical battery. The preparation method comprises the following steps of: firstly, preparing micrometer cubic blocky manganese carbonate by using a simple standing reaction method, and calcining the micrometer cubic blocky manganese carbonate into porous micrometer cubic blocky manganese dioxide; secondly, introducing a lithium salt into the porous micrometer cubic blocky manganese dioxide by a method of dissolving and stirring; and finally, carrying out calcination to obtain the porous micro cubic blocky lithium manganate material. The lithium manganate material has a regular cubic blocky structure, is uniform in grains with adjustable sizes of 0.5-5 micrometers, has favorable electrochemical reversibility and stability, and is wide in application prospect of a lithium ion power battery.