Objective: To evaluate the clinical efficacy and safety of injectable platelet-rich fibrin (i-PRF) in the treatment of mild to moderate female pattern hair loss (FPHL), and to analyze the related clinical factors affecting the therapeutic efficacy of i-PRF. Methods: This was a secondary analysis of a prospective, randomized controlled trial. A total of 29 FPHL patients with Sinclair grade Ⅱ-Ⅲ, who attended the outpatient clinics of Beijing Friendship Hospital, Capital Medical University and Peking University Third Hospital from March 2024 to April 2025, were enrolled. All patients received i-PRF treatment once every 4 weeks for a total of 4 sessions, with follow-up up to 24 weeks. Standardized clinical photographs were taken at baseline, week 12 and week 24. Trichoscopic examination was performed to measure the non-vellus target area hair count (TAHC) and hair diameter (HD) in the target area, and all adverse events were recorded. The Mann-Whitney U test was used to compare the intergroup differences in efficacy for categorical influencing factors; Spearman rank correlation analysis was employed to assess the correlation between continuous influencing factors and efficacy; and multiple linear regression (enter method) was performed to identify independent influencing factors of efficacy. Results: A total of 27 patients completed the trial. At week 12, TAHC significantly increased from 112 (105, 120) hairs/cm at baseline to 128 (110, 150) hairs/cm , with a median increase of 12 (5, 28) hairs/cm and a median growth rate of 10.5 (4.2, 24.5)% (P=0.045); hair diameter significantly increased from 42.5 (39.8, 45.2) μm to 52.0 (47.0, 58.0) μm, with a median increase of 7.5 (3.0, 12.5) μm and a median growth rate of 18.0 (7.5, 29.0)% (P<0.001). At week 24, TAHC further significantly increased to 141 (125, 158) hairs/cm (P<0.001), with a median growth rate of 24.8 (18.5, 32.0)%; hair diameter continued to improve to 55.0 (50.0, 61.0) μm (P=0.008), with a median growth rate of 21.5 (16.0, 27.0)%. The overall incidence of adverse events was 10.34% (3/29), mainly presenting as mild scalp pruritus and transient headache, without severe adverse effects. Multiple linear regression analysis revealed that baseline TAHC was an independent influencing factor for i-PRF efficacy (P<0.05), while age, BMI, disease duration, Sinclair grade, family history of AGA, and baseline HD showed no significant independent effect on efficacy in this model (P>0.05). Conclusion: This preliminary study indicates that i-PRF can improve hair density and hair shaft diameter in patients with FPHL with a favorable safety profile. Patients with FPHL and lower baseline TAHC may achieve greater clinical improvement.