Background <p>Flexible flat foot can change ground reaction forces which finally leads to lower limb overuse injuries such as tibia stress fractures and patellofemoral pain. This clinical trial aimed to evaluate the effect of long-term use of foot orthosis on vertical ground reaction force in taekwondo athletes’ girls with flexible flat foot. </p> Methods <p>Thirty eligible girls taekwondo athletes were recruited for voluntary participation in this clinical trial (age: 13.53 ± 2.16&#xa0;years, height: 149.90 ± 10.91&#xa0;cm, weight: 44.54 ± 11.44&#xa0;kg, Navicular drop: 12.96 ± 1.32&#xa0;mm). Subjects were randomly divided into experimental and control groups (15 subjects in each group). The experimental group used anti-pronation orthosis for 3 months and the control group did not receive any intervention. The outcome measurement was navicular drop, and vertical ground reaction force components during walking that were evaluated at baseline and post-intervention. Analysis of covariance was used to compare the results between groups.</p> Results <p>After 3 months of use of orthosis, there was a significant difference in the navicular drop (<i>P</i> &lt; 0.001, ηp² = 0.512), the first peak (<i>P</i> &lt; 0.001, ηp² = 0.814), the valley (<i>P</i> &lt; 0.001, ηp² = 0.738), and the second peak (<i>P</i> &lt; 0.001, ηp² = 0.720) was observed. Overall, the experimental group showed a 92% decrease in navicular drop, an 18.88% decrease in the first peak, an 8.84% decrease in the valley, and a 13.61% increase in the second peak.</p> Conclusion <p>Long-term use of orthosis can improve the vertical ground reaction force components through the correcting the foot alignment. It is recommended that taekwondo athletes with flexible flat foot use anti-pronation orthosis during taekwondo training.</p>