In our previous study, we found that modification of poly(butylene succinate) (PBS) by introducing urethane and isocyanate groups is an effective method for improving the mechanical properties and water resistance of soy-protein-based bioplastics. This study presents the effects of the structure and content of PBS components on the rheology, crystallization, and biodegradability of soy protein/PBS blends. Tests using a Haake torque rheometer and a high-pressure capillary rheometer both indicated that chemically modified PBS with a relatively low molecular weight can improve the flowability of the blend. According to differential scanning calorimeter measurements, the crystallization ability of the PBS component in a blend with improved compatibility is enhanced markedly compared with that of the pure PBS, and the nonisothermal crystallization behavior could be described by the Jeziorny model. A combination of the weight losses of the samples with the characterization of the surface microstructure shows both the structure and the content of PBS influence the biodegradation rate of the blend in a compost medium.