Before and after heat and wet treatment, the fineness and strength of PLA sample fibers were measured. Temperature, rather than moisture, was reckoned the major factor weakening the strength and fineness of PLA fiber. Acidic and alkali environments activated the hydrolysis of the fiber and lead to obvious fiber damage. Compared with the acidic environment, alkali condition resulted in far faster hydrolysis, caused more reduction of fiber diameter and strength. In addition, vertical cracks appeared on the fiber surface in alkali conditions, but did not occur in acidic environment. Finally, it was found that heat and wet treatment in a neutral environment did not make obvious harm to PLA fiber.