The invention discloses a preparation process of composite casting blanks for a high-toughness thick steel plate. The preparation process adopts continuous casting blanks with different furnace numbers, the same steel grades and different carbon contents to compound so as to meet the requirements for overall strength and core toughness, thus the composite continuous casting blanks for an extra-thick steel plate with high surface quality and excellent core impact toughness are obtained. The low carbon design of an inner layer greatly improves the metallurgical quality of the central portion andprovides strong support for improving the toughness of the center and the whole. The high carbon design of an outer layer provides a carbon source for the whole compositions to ensure the whole strength, the uniformity of the compositions is achieved through diffusion, finally the whole compositions with both toughness and strength are obtained, and the occurrence rate of surface cracks is reduced.