Biodegradable plastics, such as polylactic acid (PLA), are widely considered as sustainable alternatives to fossil fuel-derived polymers; however, their degradation generates polylactic acid microplastics (PLA-MPs), which raises ecological concerns in freshwater ecosystems. In this study, we investigated the responses of Microcystis aeruginosa to PLA-MPs (1, 10, and 50 mg/L) across a 45-day co-culture experiment. We found the PLA-MPs initially inhibited algal growth (days 18, 27) through oxidative stress, as indicated by elevated MDA content (1.94 nmol/mg protein at 50 mg/L, 1.57 times higher than the control) and SOD activity (1.30 U/mg Protein, 2.45 times higher than the control). However, during stabilization (days 36 and 45), PLA-MPs promoted growth in a dose-dependent manner, with Chl-a content in the 1, 10, and 50 mg/L groups reaching 1.12, 1.36, and 1.44 times that of the control, respectively. Concurrent shifts in inorganic carbon speciation, EPS secretion, and cellular aggregation highlighted dynamic adaptation strategies. Finally, based on the proteomic analysis, it was found that PLA-MPs promoted the growth of M. aeruginosa by improving photosynthetic efficiency, enhancing the expression of the TCA cycle and oxidative phosphorylation, and promoting cell division and proliferation. Our results confirm that PLA-MPs have a long-term effect on the growth of M. aeruginosa, providing fundamental theoretical guidance for investigating the ecological and environmental impacts of PLA-MPs in aquatic ecosystems.