The invention discloses sheet-like porous CoB powder and a preparation method thereof and belongs to the technical field of inorganic powder preparation. The powder is CoB in a crystalline state and consists of nanosheets with the average thickness being 20 nm and the plane size being 200-300 nm, and the nanosheets are connected in a staggered manner to form an open porous structure. The preparation method of the powder comprises specific preparation steps as follows: micron-order metal cobalt powder, amorphous boron powder and alkali metal chloride powder are mixed; the mixed powder is placed in an corundum crucible, heated to 800 DEG C or above under the protection of argon, kept at the temperature for 0.5-2 h and then cooled naturally; an obtained product is soaked in water, filtered, cleaned and dried, and the sheet-like porous CoB powder is obtained. The obtained CoB powder is in the crystalline state, adopts the sheet-like porous structure, can be used as a catalyst carrier material and is a potential inorganic functional material.