It is highly imperative to understand the mechanism underlying the formation of cloud-shaped defects in stainless steel to reduce the defect incidences.This study investigated the defect microstructure by SEM,simulated pickling in the laboratory,and performed structure analysis of oxide scales after hot rolling.Results show that the cloud-shaped defects in 304 No.1 surface(after hot-rolled annealing and pickling) cause the pickling chromatic aberration between the honeycomb-shaped grains of the Cr-depleted layer and the smooth grains of the stainless steel matrix,which is formed in the case of insufficient pickling.The chromatic aberration is related to the uneven oxide scales formed before pickling.With the improvement of pickling capacity,it is possible to completely pickle the Cr-depleted layer and eliminate the cloud-shaped defects. It is highly imperative to understand the mechanism underlying the formation of cloud-shaped defects in stainless steel to reduce the defect incidences.This study investigated the defect microstructure by SEM, simulated pickling in the laboratory, and performing structure analysis of oxide scales after hot rolling .Results show that the cloud-shaped defects in 304 No. 1 surface (after hot-rolled annealing and pickling) cause the pickling chromatic aberration between the honeycomb-shaped grains of the Cr-depleted layer and the smooth grains of the stainless steel matrix , which is formed in the case of insufficient pickling. The chromatic aberration is related to the uneven oxide scales formed before pickling .With the improvement of pickling capacity, it is possible to completely pickle the Cr-depleted layer and eliminate the cloud-shaped defects .