Agricultural soils are among major sinks of microplastics (MPs), which have become an alarming problem in today’s environment. While polylactic acid (PLA) is widely advertised as a biodegradable and environmentally friendly material, its persistence in natural soil raises a question whether it is really safe. This study aims to examine the sublethal effects of PLA MPs on Eisenia fetida by measuring alterations in multiple biological characteristics at four different concentrations (0.5, 1.0, 1.5, and 2.0 g/kg soil) across three exposure durations (12, 24, and 36 days). All variables studied were both concentration and time dependent. SOD and CAT activities exhibited irregular changes at different concentrations and time points. At higher concentrations and longer exposure, depletion of antioxidants was observed. Also, the neurotoxicity response to AChE activity presented a complex pattern from low to high concentrations. Higher concentrations reduced the SGR, especially at mid-term exposure time. While the reproduction was greatly decreased with increased concentrations and time. The survival rate remained at 100% until day 12 but exhibited decline during the mid and long-term studies. The tissue damage assessment revealed that the harm to tissues occurred in accordance with both dose levels and duration of exposure. The results show that PLA microplastics, which can decompose in theory, present actual toxicological hazards to earthworms when found in natural soil environments. This study demonstrates that environmental risk assessments for biodegradable plastics, as alternative to conventional plastics, require more detailed and rigorous testing before their widespread commercial implementation.