The invention provides a system and a method for predicating the amount of refining slag required during refining and desulfuration of an LF (Ladle Furnace) on line, belonging to the field of production and control of the metallurgical process. The system comprises an information acquiring module, a pool depth calculation module, a real-time temperature calculating module, a sulphur activity coefficient calculation module, a top slag quantity calculation module, a sulphur capacity calculation module, an oxygen activity calculation module, an apparent mass transfer coefficient calculation module, a sulphur distribution ratio calculation module, a real-time sulphur content calculation module and a refining slag amount calculation module. The method comprises the following steps of: respectively calculating the top slag quantity, the real-time sulphur distribution ratio, and the real-time sulphur content, and comprehensively calculating the amount of refining slag required in the desulfuration process according to the amount of molten steel, the initial sulphur content of the molten steel, the final sulphur content of the molten steel, the initial sulphur content in slag, and the final sulphur content in slag given by the initial information. According to the system and the method provided by the invention, the on-site production can be dynamically guided on line, and the defect that the refining amount can not be sufficiently and accurately calculated in real time in the prior art is overcome.