Abstract: In order to overcome the disadvantage of low activity of the ZnO nanocrystals under visible light irradiation, the six square wurtzite spong-like ZnO nanocrystals were prepared by liquid phase co-precipitation thermal decomposition method with zinc nitrate hexahydrate, hexamethylene tetramine and oxalic acid dihydrate as the raw materials. The as-prepared spong-like ZnO were composited with Ag₂O nanocrystals under alkaline condition. The obtained Ag₂O/ZnO composite photocatalysts were characterized by XRD, FTIR, UV-Vis diffuse reflection spectroscopy (UV-Vis DRS), FESEM, TEM and BET analysis. The dark adsorption and photocatalytic acivity of the Ag₂O/ZnO composite photocatalysts with different mole ratios were investigated with formaldehyde (HCHO) as a degradation model using visible light source. The results indicate that with the increase of Ag₂O, the dark adsorption of HCHO increases first and then decreases with the maximum removal of 43.34% in which the mole ratio of Ag₂O to ZnO is 1:5. On the other hand, the best degradation effect is achieved when the mole ratio of Ag₂O to ZnO is 1:10, and the photodegradation of HCHO also increases first and then decreases under visible light with 78% photodegradation of HCHO and 85% of HCHO total removal after 90 min of visible light irradiation.