Five groups of high strength concrete (HSC) specimens with a 28-day cubic compressive strength of 82.6 MPa and with fire insulation were tested, and the influence of the thickness of fire insulation on the chloride ion permeability of HSC after high temperature was discussed. Test results have shown that, in the case that the fire insulation thickness is 30 mm, the chloride ion permeability of HSC after high temperature is almost the same as that at room temperature; in the case that the fire insulation thickness is 20~25 mm, the chloride ion permeability of HSC after high temperature is still lower than that of C50 concrete at room temperature; and in the case that the fire insulation thickness is 15 mm, the chloride ion permeability of HSC after high temperature is relatively large. It can be concluded that fire insulation with a thickness of 20 mm is effective both for the inhibition of explosive spalling of HSC at elevated temperature and for the reduction of the chloride ion permeability of HSC after high temperature.