Soy protein isolate (SPI) was modified by lipid peroxidation product malondialdehyde (MDA), and the in vitro digestibility of modified SPI was investigated. Results indicated that incubation with increasing MDA concentration resulted in significant carbonyl group generation and loss of free amino groups of SPI. Fluorescence loss of natural tryptophan and formation of Schiff base were observed. Noncovalent interaction between molecules was enhanced and became the main force that led to the solubility reduction of MDA-modified SPI. Differential scanning calorimetry (DSC) indicated that SPI had higher thermal stability and lower total calorimetric enthalpy after MDA pretreatment. Electrophoresis showed that β-conglycinin was more sensitive to MDA modification. In vitro digestion indicated that MDA could induce non-disulfide covalent polymer of SPI, which could not be digested by pepsin and pancreatin. β subunits of β-conglycinin became more resistant to digestion with increasing MDA concentration. Evaluation of the free amino acid profile in the digests indicated that MDA-modified SPI had deteriorating nutritive quality.