The effects of extrusion temperature and whey protein concentrate (WPC) addition on the physical properties of extruded corn starch were studied. The correlations between the physical properties of the extrudates and the internal chemical bonds in the protein were analyzed using the statistical method of Pearson. Increasing the temperature from 130 °C to 150 °C resulted in lower specific mechanical energy (SME); expansion ratio (ER); water solubility index (WSI); protein solubility in phosphate buffer, 2-mercaptoethanol and sodium dodecyl sulfate (Pb + 2-Me + SDS); and a higher hardness of the extrudates. On the other hand, increasing the level of WPC (0–40%) resulted in a lower SME, WSI, content of sulfhydryl (SH) groups, degree of gelatinization (GD), protein solubility in Pb + 2-Me + SDS, and a darker color of the extrudates. The ER at 130 °C first increased from 8.80 to 14.04 and then decreased to 6.86 with the increasing WPC content from 0 to 40%, whereas the trend of hardness was the opposite. Correlation analysis revealed that the content of SH groups was significantly related to ER, SME, WAI, WSI, GD, and color, which may be a good indicator for the quality of whey protein-fortified extrusion-expanded products.