During the rolling of ultra-thick composite plate,cracks are found in the cladding. Through the metallo-graphic test,it is determined that the crack source comes from the joining surface. Through the microscopic metallographic and hardness test of the joining surface,it is analyzed that the reason of the crack source on the joining surface of the com-posite plate and the cracks in cladding caused by crack sources during the rolling is mainly due to insufficient explosion parameter design of the composite plate,both the strength and toughness of the joining surface are low,and the times of leveling and pressure of ultra-thick during the leveling increase,which leads to large cracks and cavity defects in the cladding on the joining surface,and the defects relatively concentrate,and the cracks expand in multiple directions;in addition,the design of heat treatment parameters after explosive is insuffi-cient,which causes the hardness of the cladding on the joining surface is much higher than that of the base metal and the cladding.