The invention relates to a method for rolling a liquid core under heavy reduction rate by virtue of an online thickness regulating roll by utilizing a continuous casting machine. The method comprises the following steps of: when two rolling mills are loaded, setting a target thickness, feeding back the thickness value in real time and detecting oil pressure, carrying out conversion on a rolling force to judge whether the rolling force is higher than a limit value, if the rolling force is lower than the limit value, stabilizing rolling until the rolling is finished; if the rolling force is higher than the limit value, regulating and judging the rolling force again; if the target thickness is changed in the rolling process, calculating thickness variation by virtue of a rolling process computer, regulating roll gap of the two rolling mills to be the target thickness, and judging whether the rolling force is higher than the limit value again. In the method provided by the invention, a rolling signal can be changed to change the target thickness in the rolling process, and a wedge-shaped blank is rolled. By applying the method provided by the invention, the quality of a continuous casting can be better improved, the continuous casting and the wedge-shaped blank can be rolled, and the time of a process is reduced while the yield is improved; meanwhile, structure of equipment is simple, investment is less, control is simple, reliability is high, and energy can be greatly saved, thus the method has a good popularization and application prospect.