The invention discloses a preparation method of high-purity MoAlB ceramic powder and a compact block body. A method for preparing MoAlB powder is characterized in that Mo powder, Al powder and B powder are used as raw materials; proportioning is performed according to the mole ratio of Mo to Al to B being 1:(1.1 to 1.3):1; dry blending is performed for 5 to 24h in a ball grinding mill; after the mixed materials are subjected to blank pressing on a tableting press; blanks are put into a pressure-free furnace; under the argon gas atmosphere, the furnace temperature is raised to 1000 to 1300 DEG C at the velocity of 10 to 50 DEG C/min; heat insulation is performed for 10 to 120min; then, the temperature is lowered to room temperature; the high-purity MoAlB material is obtained. The obtained MoAlB material is crushed in the ball grinding mill and is sieved by a sieve with different meshes, so that the MoAlB powder with different particle size is obtained. A method for preparing the compact block body is characterized in that Mo powder, Al powder and B powder are used as raw materials; proportioning is performed according to the mole ratio of Mo to Al to B being 1:(1.1 to 1.3):1; dry blending is performed on the material for 5 to 24h in the ball grinding mill; mixed materials are put into a pressure-free furnace; under the argon gas atmosphere, the furnace temperature is raised to 1000 to 1300 DEG C at the velocity of 10 to 50 DEG C/min; pressurization is performed to 10 to 40MPa; heat insulation is performed for 10 to 120min; then, the temperature is lowered to the room temperature; a high-purity compact MoAlB block body material is obtained.