Pepsin was used for in vitro simulated digestion of heat-treated soybean protein isolate （SPI）. The subunit composition， protein secondary structure and molecular weight distribution of digestion products were researched. The degree of hydrolysis increased firstly and then decreased with the increase of heat treatment temperature and time， which indicated that proper heat treatment conditions could promote the digestion and degradation of SPI by pepsin. The heat treatment was able to change in vitro digestion mode of SPI， reducing the digestion degree of 11S subunit， leading to the digestion and degradation of 7S subunits， which formed the component with the molecular weight lower than 17 ku. Heat treatment mainly resulted in the increase of α-helix content and the decrease of β-sheet content in the secondary structure of digestion products. The molecular weight distribution of SPI digestion products was also changed. As rising of heat treatment temperature， the α-helix content increased firstly and then decreased， whereas the opposite trend was noted for β-sheet content. However， the heat treatment time had no significant effect on the content of all secondary structures. For the SPI digestion products， high temperature or heat treatment for long time could increase the distribution in the range of more than 100 ku and 3-10 ku， while decrease the distribution in the range of 10-100 ku and less than 3 ku.