The BIBA (Busan Ion Beam Accelerator) is a compact linear accelerator facility using the 28 GHz ECRIS (Electron Cyclotron Resonance Ion Source) at the KBSI (Korea Basic Science Institute). The superconducting magnets of the 28 GHz ECRIS produce high magnetic fields for strong confinement of plasmas in an ion source chamber. For stable operation of the superconducting magnets, performance of cryostat system is very essential. However, the cryostat system produces significant quantities of the heating due to conduction from room temperature. In addition, part of the X-ray radiation produced by the collisions of the electrons within the ion source chamber is absorbed by the cold mass of the superconducting magnet, leading to an additional heat load in the cryostat system. In this paper, a study on the performance of the cryostat system for 28 GHz ECRIS is performed to improve the cooling efficiency of the cryostat system.