The invention discloses a preparation method of hierarchical-pore polymer solid acid capable of efficiently adsorbing low-concentration ammonia gas. The preparation method for the solid acid comprisesthe following steps: with an organic solvent and deionized water as solvents, divinylbenzene and sodium p-styrenesulfonate as monomers, and 2,2'-azobisisobutyronitrile as an initiator, carrying outa solvothermal copolymerization reaction and evaporating the solvents to obtain an intermediate product; and subjecting the intermediate product to ion exchange with an acid, and performing drying toobtain the hierarchical-pore polymer solid acid. The solid acid has a relatively large BET surface area, a layered nanopore structure and strong acidity, is controllable in structure and easy to operate, and can be produced on a large scale; meanwhile, the solid acid has excellent adsorption performance on ammonia gas, the NH3 adsorption capacity of the solid acid reaches 9.29 mmol/g under the room temperature condition of 1.0 bar, and the N2 adsorption capacity and the H2 adsorption capacity of the solid acid under the same condition are only 0.03 mmol/g, so the performance of the solid acidis superior to the performance of similar ammonia gas adsorption materials. The solid acid has good application prospects in selective adsorption separation of ammonia gas in industrial waste gas.