The invention relates to a preparation method of an anhydrous magnesium sulfate drying agent. The preparation method comprises a workbench, a first processing device and a second processing device, the upper end of the workbench is fixedly provided with the first processing device, and the upper end of the first processing device is fixedly provided with the second processing device; the first processing device comprises a cylindrical plate, a vibration motor, a screening frame, a first motor, a rotating column, a grinding disc, a first grinding ring, a second grinding ring, a screening disc and a discharging pipe, the first processing device is matched with the second processing device, anhydrous magnesium sulfate blocks are minced, and minced anhydrous magnesium sulfate particles are screened and ground; and finally, the ground anhydrous magnesium sulfate powder is screened and collected, so that the quality of the anhydrous magnesium sulfate powder is uniform, meanwhile, the production efficiency of equipment is improved, the abrasion loss of grinding parts during grinding of anhydrous magnesium sulfate particles is reduced through the first processing device, and the surface stress of the grinding parts is uniform.