Abstract The Koreas Superconducting Tokamak Advanced Research (KSTAR) PF cryo-circuit is designed for cooling the fourteen superconducting magnets (Nb3Sn, NbTi) and structures. Those are cooled down by the supercritical helium (4.5K and 5.5bar) of a forced flow (pressure gradient: 2bar) in order to maintain the supercritical state of the helium. To supply a large amount of supercritical helium (>370g/s), a circulator was inserted into the PF cryo-circuit. The compressed supercritical helium is distributed to five helium manifolds with cryogenic valves and supplied to each PF magnet. While the PF magnets had been operating, the mass flow rate reduced and the pressure head of the circulator was fluctuated depending on the PF magnet operation scenario. These phenomena could damage the circulator and could stop it during operation. Therefore, by-pass valve, which is parallel with in-line valve and is connected with inlet and outlet of the magnet, was opened in order to reduce of the circulator's pressure head. In this paper, we focused on the helium behavior of the superconducting magnet when the by-pass valve was opened in order to release the pressure head of the circulator and the results will be presented.