Objective To observe the clinical effect and safety of poly-L-lactic acid in improving the skin relaxation of mandibular margin. Methods A total of 26 patients with mandibular margin skin relaxation who were treated in our hospital from December 2022 to January 2023 were selected as the research objects. In this study, poly-L-lactic acid and type A botulinum toxin were injected on both sides of the mandible by self-control method. The control side was treated with type A botulinum toxin, and the observation side was treated with poly-L-lactic acid injection. The improvement of MAS score, incidence of adverse reactions and patient satisfaction were compared between the two sides. Results The improvement value of MAS score on the observation side was (1.31±0.47) scores, which was higher than (0.92±0.27) scores on the control side, and the difference was statistically significant (P