Dimethyl sulfoxide (DMSO) was added to the starch by melting processing. It was studied respectively that DMSO content effected on mechanical properties, flowabilities, water resistance properties, scanning electron micrographs (SEM) of DMSO-starch (TPS)/DMCS-soy protein isolates (DMCSSPI) materials. SEM results indicated that the crystalline of starch is destructed and the starch granules transferred to a continuous amorphous phase. It was found that the DMSO could more effectively restrain the retrogradation than the plasticizers, due to the stronger hydrogen bonds. For TPS/DMCSSPI composites, the increase of TPS content could make its water resistance properties worse. However, with DMSO content increase, its elongation at break and toughness could be enhanced.