The invention relates to a method and a system for determining the steel-tapping quantity of a converter, which consider that the working environment of the steel-making process of the converter is severe, the measurement is difficult and the interference of other factors is large, and provide a data-driven prediction model based on data, combine a Principal Component Analysis (PCA) with a RBF neural network, find the relation and the internal relation among variables by carrying out mathematical analysis on the related internal structure of the original variables, can quickly and accurately realize the prediction of the steel-tapping quantity of the converter, improve the component hit rate and the product stability in the steel-making process of the converter, are beneficial to realizing the control of narrow regions of steel-making components, save the alloying cost and have good application prospects in the field of ferrous metallurgy.