The soybean protein fiber is a new textile material. Thanks to its fine denier, light specific gravity, high strength and silk-like luster，it has so excellent cashmere handle, silk softness and heat resistant properties that it has been called manmade cashmere and found wide application. In order to understand the relationship between the inner structure and the mechanical property and the effect of bleaching and dyeing to the soybean protein fibers，four mechanical models are chosen. The tensile and relaxation property of the soybean protein fibers are analyzed. The tensile and relaxation cures are fitted with the suitable model. It shows that the relaxation property of SPF is in accordance with the standard linear solid model. Estimates of the Hookean spring modulus at 8% and at 10% are different, so some structural modifications could be produced by the strain. Bleached fibers show a higher level of relaxation than raw fibers and dyed fibers. Bleaching has a remarkable influence on decreasing tenacity at break for each test modality. Knotted and looped modalities decrease fiber tenacity remarkably in all three samples.