Polylactic acid (PLA) is a biodegradable alternative to petroleum-based plastics, but its slow natural degradation rate and underlying mechanisms remained poorly understood. In this study, the properties of PLA degradation by Bacillus sp. JA-4 were investigated, and the molecular mechanisms involed in PLA degradation were elucidated through RNA sequencing (RNA-seq) analysis. Scanning electron microscopy (SEM) revealed biofilm-induced surface erosion, leading to the formation of cracks and holes in the PLA film. The analysis of Fourier-transform infrared spectroscopy (FTIR) and liquid chromatography-mass spectrometry (LC-MS) confirmed ester bond cleavage (1260 cm⁻, 1127 cm⁻, and 1080 cm⁻) during PLA degradation, and the degradation intermediates including lactic acid monomers and five lactic acid oligomers were identified. RNA-seq analysis indicated that there were 360 upregulated genes associated with environmental adaptation and energy metabolism, likely involved in the degradation process. The addition of gelatin, sodium dodecyl sulfate (SDS), yeast powder, peptone, and casein significantly enhanced PLA degradation, with gelatin being the most effective inducer. By the 10th day, weight loss of 23.1% of PLA and protease activity of 31.6 U/mL were achieved at a gelatin concentration of 3%, which were higher than the control group. This study provides a novel microbial resource and theoretical foundation for the degradation of residual PLA in the environment.