Shear and extensional properties of high-density polyethylene (HDPE) and poly(butylene succinate) (PBS) blends were measured using capillary rheometers to investigate the influences of processing conditions and mixing ratios on flow behaviors, and the predictive abilities of extensional master curve, back propagation neural network model, and constitutive equation were analyzed for the extensional flow. The results showed that the shear flow of blend melts obeyed the power law, and the influence of PBS content on shear-thinning behavior was obvious. The extensional viscosity of the blends decreased with extensional strain rate, extrusion rate, and PBS content increasing. The extensional master curve was applied for HDPE/PBS blends, and scaling factor reduced with the increasing extrusion rate. In addition, the approach of back propagation neural network model was an effective approach to predict extensional viscosity for the purposes of polymer processing control and monitoring.