The effect of additional pulmonary blood flow (APBF) on the hemodynamics of bilateral bidirectional Glenn (BBDG) connection was marginally discussed in previous studies. This study assessed this effect using patient-specific numerical simulation. A 15-year-old female patient who underwent BBDG was enrolled in this study. Patient-specific anatomy, flow waveforms, and pressure tracings were obtained using computed tomography, Doppler ultrasound technology, and catheterization, respectively. Computational fluid dynamic simulations were performed to assess flow field and derived hemodynamic metrics of the BBDG connection with various APBF. APBF showed noticeable effects on the hemodynamics of the BBDG connection. It suppressed flow mixing in the connection, which resulted in a more antegrade flow structure. Also, as the APBF rate increases, both power loss and reflux in superior venae cavae (SVCs) monotonically increases while the flow ratio of the right to the left pulmonary artery (RPA/LPA) monotonically decreases. However, a non-monotonic relationship was observed between the APBF rate and indexed power loss. A high APBF rate may result in a good flow ratio of RPA/LPA but with the side effect of bad power loss and remarkable reflux in SVCs, and vice versa. A moderate APBF rate could be favourable because it leads to an optimal indexed power loss and achieves the acceptable flow ratio of RPA/LPA without causing severe power loss and reflux in SVCs. These findings suggest that patient-specific numerical simulation should be used to assist clinicians in determining an appropriate APBF rate based on desired outcomes on a patient-specific basis.