Abstract: Iron ion is one of the most important radiations in cosmic environment, because its equivalent dose and biological effects pose a threat against astronauts. Geant4 toolkit is used for simulating tissue equivalent proportional counter response to iron ion spectra from Galactic Cosmic Rays. It is found that the linear energy of iron ion in galactic cosmic rays is between 80 keV/μm and 600 keV/μm and the maximum is between 100 keV/μm and 150 keV/μm, accordingly; its quality factor is more than 25, which means very high biological effects.