The invention provides a method for on-line prediction of total decarbonization amount of molten steel in a circulating vacuum degassing method. The method comprises steps of: dividing a decarbonization process in a circulating vacuum degassing device into a high-speed reaction stage, a forced decarbonization stage and low-concentration reaction stage; segmenting the decarbonization positions in the circulating vacuum degassing device; and calculating a sum of the respective decarburization amount of the decarbonization positions corresponding to the high-speed reaction stage, the forced decarbonization stage and the low-concentration reaction stage, so as to obtain the total decarbonization amount of the molten steel. The method provided by the invention has the advantages of capability to carry out real-time detection on the total decarbonization amount of the molten steel in an RH process, flexible operation, high precision and low cost.