The cryogenic circuit used to cool a large superconducting magnet such as a tokamak system must be designed while considering the cooling conductance due to the many branches.The KSTAR PF1~2 upper and low magnet have ten and eight cooling channels parallel respectively. The pressure drop of the magnets is adjusted by cryogenic valve and is maintained by a supercritical helium circulator. The flow rate should be uniform among the cooling channels or magnets but the flow unbalance is observed in the real system. To investigate of the unbalance effects, the simple model of PF1~2 upper and low magnets is developed using Supermagnet code. The maximum temperaure is studied in details depending on the unbalance ratio.