Abstract: The use of direct rolling process of the continuous casting billet instead of die casting or composite stitch welding process to produce high-quality extra-thick plates can greatly improve production efficiency, yield, and reduce production costs and energy consumption. The formation mechanism of the internal defects of the extra-thick plate produced by conventional process was studied, and it was revealed that during the continuous casting process, the final solidification points at both sides of the width direction lead to dense porosity and shrinkage in the 350-600 mm zone from the sides of the slab, and it cannot be pressed during subsequent rolling, resulting in unqualified flaw detection and Z-direction performance. In order to solve the loose porosity and shrinkage inside the casting billet, a new patented technology of large roll diameter and large reduction was applied for the first time. On the continuous casting line,implement of large-diameter roll reduction under the large gradient temperature field（＞400 ℃） formed at the end of solidification or just after complete solidification can significantly improve the central defects of the thick slab. Based on the continuous casting slab with large reduction, a high-permeability rolling technology with low speed, low-temperaturecontrolled thickness and rough rolling reduction was developed, and the slab rolling test containing drilling was designed to characterize rolling permeability. Using the above technology, combined with smelting and heat treatment technology, the development of high-quality extra-thick Q355 EZ35 plate products with a thickness of 200 mm has been successfully realized by using the 380 mm thick continuous casting slab, which guarantees flaw detection and performance.