Abstract: The reason of cracks was studied by analyzing the surface crack of slabs and experimental research in this dissertation.Result shows: surface cracks on CCSD36 steel mainly result from the original slab cracks.It originates in mold and extends in secondary cooling zone and rolling.Oxidation layer along the cracks formed in high temperature makes cracks hard to be welded.A large amount of inclusions with high melting point and low plasticity,such as silicates,exist inside and around the cracks,the content of which is deviated from the normal state seriously,and brittle particles are formed.When they are captured by exterior and oxidation layer,the surface will break during being rolled,so the cracks extend or newly formed.