The melt flow rate (MFR) is a crucial quality indicator for polyolefin products. However, the offline laboratory measurement method suffers from significant time delays between sampling and obtaining test values, making it inconvenient for real-time product quality control. Additionally, installing an online analyzer can be expensive. In this study, we propose a soft sensor method that combines mechanism analysis and data-driven approaches to predict the MFR in the extruder process. Firstly, we analyze the physical mechanism of the extruder process, and the time series data are processed into independent enhanced data sets by reasonably selecting data enhancement parameters. Secondly, we statistically model the distribution of particle MFR and classify the data into four sections. We train Deep Neural Network (DNN) and Gradient Boosting Decision Tree (GBDT) regression models separately for low and non-low MFR sections while also developing a global classification model to invoke corresponding regression models when needed. The results demonstrate that our classification model achieves 100% accuracy in distinguishing between low and non-low MFR categories; furthermore, our regression models achieve an RMSE of 0.600, and an R2 value of 0.998, outperforming existing models on various indicators. Finally, considering viscosity variations in polyolefin products' influence on prediction performance, we recommend setting data enhancement parameters contrary to particle MFR trends.