Hypoplastic left heart syndrome is currently the most lethal cardiac malformation of the newborn infant. Survival following a Norwood operation depends on the balance between systemic and pulmonary blood flow, which is highly dependent on the fluid dynamics through the interposition shunt between the two circulations. The purpose of this study is an optimization of the systemic-to-pulmonary artery shunt. In this study, We used computational fluid dynamic(CFD) models to determine the velocity profile in a systemic-to-pulmonary artery shunt and suggested a simplified method of calculating the blood flow in the shunt based on Ultrasound systems. We analyzed the flow characteristic variations and oscillatory shear index(OSI) due to the anastomosis angle and shunt diameter changing. Four different CFD models were constructed with the shunt sizes ranging from 3 to 3.5mm. The angle between the brachiocephalic trunk(BCT) and the shunt were $30^{circ}$ and $45^{circ}$, respectively. When the diameter is 3.0 mm, the oscillatory shear index decreased by 1.2% at $30^{circ}$ as opposed to at $45^{circ}$. When the diameter is 3.5 mm, it increased by 18% more at $30^{circ}$ as opposed to at $45^{circ}$. When the joint angle is $30^{circ}$ and the diameter is 3.0 mm, the oscillatory shear index decreased by 4.1% in comparison with the 3.5 mm diameter. When the angle is $45^{circ}$ and the diameter is 3.0 mm, the index increased by 14.6% in comparison with the 3.5 mm diameter.