Acute radiation dermatitis (ARD) is the most common treatment-related adverse reaction following radiotherapy after modified radical mastectomy and/or prosthetic breast reconstruction in breast cancer patients. Moderate-to-severe ARD may compromise cosmetic outcomes and quality of life, and even impair radiotherapy efficacy. Current pharmacological prophylactic measures clinically employed - including topical corticosteroids, superoxide dismutase, and trolamine cream - demonstrate suboptimal efficacy and lack high-level evidence-based medical support. Emerging research indicates an association between cutaneous microbial homeostasis and ARD development, suggesting that maintaining skin surface acidity and modulating microecological balance may represent more effective preventive strategies. This study evaluates the efficacy and safety of a natural weak-acid macromolecular/small molecular repair cream in preventing ARD among post-operative breast cancer patients receiving radiotherapy through skin microbiome modulation. We enrolled 326 high-risk early-stage breast cancer patients scheduled for post-operative radiotherapy and randomized them to compare the superiority of the natural weak-acid repair cream versus conventional care with moisturizer. Concurrent skin microbiome sampling was performed to assess microecological changes and their impact on ARD development. The findings will provide high-level clinical evidence and theoretical basis for the safer and more effective application of this natural weak-acid repair cream in preventing post-radiotherapy ARD in breast cancer patients.