The invention relates to the field of smoke denitration, in particular to a method for fully recovering silicon dioxide in a waste SCR (Selective Catalyst Reduction) catalyst. The method comprises the following steps: soaking the pulverized waste SCR catalyst with three to four times of water to remove impurities, leaching powder from which the impurities are removed with excessive thick alkaline solution with the concentration of 60-80wt%, filtering to obtain a Na2TiO3 solid, adding sulfuric acid into filtrate, regulating pH to 11.5-12.5 at a temperature of 70-90 DEG C, leaving the mixture to stand for 0.5-1h, filtering, and discarding filter cakes; adding sulfuric acid into the filtrate until the pH is regulated to 9-10 to obtain the silicate filter cakes, heating and stirring the silicate filter cakes in mixed acid of dilute sulfuric acid and oxalic acid at a temperature of 40-60 DEG C for 1-2h, filtering, washing the filter cakes with deionized water, washing off acid radical ions in the solid, and obtaining high-purity silicon dioxide through heating and decomposition. Therefore, the recovery of the silicon dioxide in the waste SCR catalyst can be realized without a roasting process.