The invention relates to a synthesis method for hierarchically porous ZIF-8 and a preparation method for deep desulfurizing agent. The method includes the following steps: 2-methylbenzimidazole is dissolved in sulfuric acid, the concentration of which is 98 wt percent, hydrogen peroxide, the concentration of which is 30 wt percent, is then dripped, the solution is heated to a certain temperature and refluxed under constant temperature for 2 to 5 hours after dripping, the reaction solution is poured into ice water and recrystallized, and product is separated out, washed and dried, so that 2-methylimidazole-4, 5-dicarboxylic acid is obtained; the 2-methylimidazole-4, 5-dicarboxylic acid as ligand, template and sodium hydroxide are dissolved into deionized water, uniformly mixed and then added with zinc source, and after agitation, mixed solution is obtained; the mixed solution is crystallized, and solid product is then separated out, washed and dried, so that ZIF-8 raw powder is obtained; with organic solvent as extracting agent, the template in the ZIF-8 is extracted out, and thereby the hierarchically porous ZIF-8 is obtained. The invention also provides the method for preparing the deep desulfurizing agent by utilizing the hierarchically porous ZIF-8. The hierarchically porous ZIF-8 synthesized by the method has the characteristics of high external specific surface area and high pore volume.