Abstract: The formation mechanism and influencing factors of fold defects at the medium plate edge were studied by detecting and analyzing edge folding samples. It is found that the plate edge folding phenomenon results from the transformation of slab lateral side to plate surface during the broadside rolling process. The microstructure observed at the fold defects is composed of iron oxide and a decarburization layer formed at high temperature in the plate surface before rolling. Finite element numerical simulation of the rolling process confirms that the folding defects are mainly caused by the transformation of lateral side. The influences of slab defects,rolling procedures,broadening passes and reduction on the plate fold defects were obtained by rolling experiments. Experimental results show that rolling spread leads to the fold edge defects,which do not depend on the quality of the slab. In the process of rolling,the folding of lateral side transfers to the plate surface and changes to be edge seam defects. The distance of folding to the edge increases with the increment of broadside rolling passes and reduction.