Soy protein plastic sheets were made by extrusion. The effects of water, glycerol, methyl glucoside, ZnSO4, epichlorohydrin, and glutaric dialdehyde on the mechanical properties of soy protein plastic sheets were studied. The thermal transition temperatures and dynamic mechanical properties of soy protein plastics were also investigated. Depending on the moisture and glycerol contents, soy protein plastic sheets displayed properties from rigid to soft. The glass transition temperatures of the sheets varied from ca. −7 to 50°C with moisture contents ranging from 26 to 2.8% and 30 parts of glycerol. The soy protein plastic sheets were usually in their glassy states at room temperature unless they contained high moisture. The β-transitions of soy protein plastic sheets ranged from −33 to −72°C depending on the moisture. After being submerged in water for 20 h, the soy protein sheets absorbed up to 180% water. With the presence of two parts of ZnSO4, water absorption of the soy protein sheets decreased by 30%. Soy protein sheets absorbed or lost moisture depending on the relative humidity of the environment.