The invention discloses an online cleaning method for recovering the activity of a low-temperature denitration catalyst, and mainly aims to eliminate influence on the activity of the catalyst and simultaneously avoid adhesion of ammonium sulfate to a downstream flue. The online cleaning method for recovering the activity of the low-temperature denitration catalyst comprises the following steps: 1, performing gas blowing on the catalyst on a catalyst bed for preliminary dust cleaning; 2, performing liquid spraying washing on the catalyst treated by Step 1 to remove impurities. According to the method, the catalyst on the catalyst bed is treated to recover the activity, so that the denitration efficiency of the catalyst bed can be kept stable for a long time, and meanwhile, adhesion of the ammonium sulfate to the downstream flue can be avoided; operating temperature in the whole flow does not exceed normal running temperature of the catalyst, so that damage to original activity of the catalyst is prevented; moreover, a cleaning process is implemented on line without influence on normal running of denitration equipment, and the method has the advantages of small flue gas fluctuation, low energy consumption and the like.