The invention provides a preparation method of a porous fly ash adsorbent and application of the adsorbent. The method takes metal oxide Fe2O3 and MgO in coal combustion fly ash which is used as an adsorbent matrix and effectively components for oxidizing and adsorbing HgO; while fly ash aggregate is prepared, CaO is changed into Ca(OH)2 and the porosity of the fly ash aggregate is also increased;the application is that the adsorbent is used for removing the HgO in coal combustion flue gas in a high-precision manner in a temperature range of 80 to 300 DEG C until the content of the HgO is 4mug/m<3>. The adsorbent prepared by the method has a relatively good pore structure and the specific surface area is greater than 180m<2>/g; the fly ash, corncobs, sodium bentonite and sodium carboxymethyl cellulose are cheap and easy to obtain; a preparation technology is simple and feasible; the adsorbent has relatively good mechanical strength which is greater than 60N/mm<2>, and can be widely used for removing the HgO in various coal combustion flue gases.