The pore structure of ultra-high performance concrete (UHPC) was analyzed by nitrogen adsorption and desorption (NAD) test in this paper. The influence of steel fiber and mineral admixture on the chloride ion transport behavior of UHPC under different hydrostatic pressures were investigated. Scanning electron microscope and energy dispersive spectrometer analysis were conducted to examine the microstructure of UHPC. The results show that the addition of steel fiber increases the porosity of UHPC matrix, while the variation is not obvious. The addition of slag reduces the porosity of matrix. When the early steam curing is followced by standard curing to 28 d, a large amount of fly ash increases total porosity. The chloride ion concentration and apparent chloride diffusion coefficient at the same depth increase with the increment of hydrostatic pressure, which is more significant under the pressure of 2 MPa. It is found that the linear relationship exist in the content of free chloride ions and the content of total chloride ions. The incorporation of steel fiber reduces the chloride binding ability, while the incorporation of slag and fly ash effectively improves the binding rate of chloride ion, and the maximum chloride binding rate reaches 46.29%. Scanning electron microscope and energy dispersive spectrometer observation suggest that the corrosion of steel fiber only occurs on the surface of UHPC.