Abstract: In order to improve the degradability of polylactic acid (PLA), bamboo fiber (BF) and sodium alginate (SA) were blended with PLA to prepare composite materials. And then the PLA composites were buried in soil for degradation test. The mass loss, morphological properties, FTIR, thermal properties and crystallinity of the PLA composites were measured after degradation. The results shown that both SA and BF can improve the mass loss of PLA composites during degradation. After 21 days of degradation, the mass loss rates of BF/PLA and SA-BF/PLA composites were 0.83% and 2.54%, which were 7.55 and 23.09 times higher than that of pure PLA (0.11%), respectively. A large number of cracks and depressions appeared on the surface of SA-BF/PLA composites after degradation, which increased the contact area between the PLA composites and the soil, and thus accelerated the degradation of the PLA composites. The mass loss rate of PLA materials was very low after degradation, but the carbanyl group of PLA materials increased significantly, this indicated that soil degradation could lead to the fracture of some PLA long-chain polymers into small molecules. Compared with pure PLA, the crystallinity of BF/PLA and SA-BF/PLA composites was significantly reduced, this meant that SA and BF could reduce the crystallinity of the PLA composites and enhance their degradability. The results indicated that SA and BF can improve the degradability of PLA composites. The results would provide theoretical reference for the preparation of highly degradable PLA composites.