Prediction of melt index (MI), the most important parameter in determining the product's grade and quality control of polypropylene produced in practical industrial processes, is studied. A novel soft-sensor model with principal component analysis (PCA), radial basis function (RBF) networks, and multi-scale analysis (MSA) is proposed to infer the MI of manufactured products from real process variables, where PCA is carried out to select the most relevant process features and to eliminate the correlations of the input variables, MSA is introduced to acquire much more information and to reduce the uncertainty of the system, and RBF networks are used to characterize the nonlinearity of the process. The research results show that the proposed method provides promising prediction reliability and accuracy, and supposed to have extensive application prospects in propylene polymerization processes.