Introduction: The workloads of baseball pitchers are significant risk factors for injury in adolescent players. Increased pitching volumes and repetitive throwing may result in overuse injuries and alter the pitching mechanism, increasing the risk of pitching-related injuries to the elbow and shoulder. This study analyzed the effects of accumulated pitching volumes and changes in the pitching mechanism's biomechanical parameters among adolescent baseball pitchers. Methods: We recruited 36 teenage male right-handed baseball pitches who use an overhand delivery. After warm-up, each subject performed five pitches as the pre-test data collection. Each participant then pitched five simulated innings, consisting of 20 pitches per inning with 8 minutes of rest between innings. Each participant threw 100 total pitches, and the last five pitches were used as the post-test data. A motion analysis system with six cameras (Motion Analysis Corp, 250 Hz) was used to collect 3D reflective marker trajectories, and joint kinematics were calculated. A force plate (Kistler, 2500 Hz) was used to collect ground reaction force (GRF) data from the stride leg. Ball speed was measured by pocket radar. Muscle fatigue levels were assessed by self-reported rating of perceived exertion (RPE) and visual analog scale (VAS). A paired t-test was used to compare differences between pre-test and posttest pitching biomechanics parameters. Results: The results showed that pitching velocity was significantly decreased (p = .001), and both RPE and VAS were significantly increased (p < .001) after pitching 100 balls. Alterations in pitching kinematics were primarily noted at the pelvis, trunk, and upper arms during the acceleration phase of the pitching cycle. The vertical GRF of the stride leg at the point of ball release significantly decreased (p = .012), and anterior-posterior momentum significantly increased (p < .001). Conclusion: Teenaged baseball pitchers became fatigued after repetitive pitching, and pitching velocity decreased as pitching volume accumulated. In addition, compensatory alterations to the pitching mechanism following fatigue might increase the potential risk for upper arm injuries. The pitching volumes of teenage pitchers should be monitored during games and daily training. Future studies should include joint kinetics during pitching to effectively discuss the related risk of injury.