Objective: To analyze the effect of using red-light and blue-light combined with alpha-hydroxy acid in preventing post-cystic scar in cosmetic dermatology. Methods: Fifty patients who received acne treatment in our hospital from January to November 2025 were selected as the research subjects. They were divided into the control group and the study group by random number table method, with 25 cases in each group. The control group received comedone extraction and red-light and blue-light irradiation treatment, while the study group received additional alpha-hydroxy acid treatment on the basis of the control group. The scar retention rate, adverse reaction occurrence rate, treatment satisfaction, and skin smoothness improvement of the two groups after treatment were compared. Results: The scar retention rate of the study group (20.00%) was significantly lower than that of the control group (60.00%) (P < 0.05); there was no significant difference in the adverse reaction occurrence rate (20.00%) and treatment satisfaction (96.00%) between the study group and the control group (12.00% and 72.00%, respectively) (P > 0.05); the treatment satisfaction of the study group (96.00%) was significantly higher than that of the control group (72.00%) (P < 0.05); the skin smoothness improvement of the study group was significantly better than that of the control group (P < 0.05). Conclusion: The use of red-light and blue-light combined with alpha-hydroxy acid in the intervention for preventing post-cystic scar in cosmetic dermatology can significantly reduce the scar formation rate of patients and improve skin smoothness. It is an effective intervention method and is worthy of clinical application.