Purpose: This study aimed to investigate the patterns of breast development and the factors influencing this process following Gender-Affirming Hormone Therapy (GAHT) in Chinese transgender women. Methods: The clinical data of 113 transgender women from July 2022 to July 2024 were retrospectively analyzed. In addition, in the prospective longitudinal study, 42 transgender women were followed at 4 time points: 0, 3, 6, and 12 months after starting GAHT. Demographic characteristics, Body Mass Index (BMI), Body Fat Percentage (BFP), serum estradiol level, and breast thickness measured by ultrasound were collected. SPSS 25.0 and Python software were used for statistical analysis, and genetic algorithm was used to analyze the related influencing factors. Results: With the extension of GAHT, mammary gland thickness gradually increased, with the most rapid development occurring during the first six months and stabilization after two years. Mammary gland thickness was positively correlated with treatment duration, BMI, BFP, and serum estradiol levels, with treatment duration having the greatest impact. Over the two years of GAHT, the genetic algorithm predicted that maximum mammary gland thickness (16.35 mm) would be achieved in approximately 22.53 months, corresponding to a BFP of 24.96% and a serum estradiol level of 129.30 pg/ml. Conclusions: After receiving GAHT, breast development in transgender women was most pronounced during the first six months and stabilized after two years. Increased breast development correlated with treatment duration, BMI, BFP, and serum estradiol levels, providing clinical reference values for treatment optimization. Further studies are necessary to validate long-term outcomes.