Abstract: To explore the thermal-hydraulics characters of Pb-Bi fast reactor sub-channel, the sub-channel program SACOS-PB was developed independently. As an example, the rectangle nine fuel rods construction was set, and its coolant temperature along the axial length was calculated. The results show that calculating results are good agreement with the reference results and the CFX calculating results. The three different constructions of the sub-channels were analyzed using SACOS-PB program, and the results show that the hexagon construction is the most suitable construction. The current research results establish a foundation for the further study.