To explore the attenuation rules and mechanism of impermeability of basalt fiber reinforced concrete under dynamic fatigue load, the best parameters of basalt fiber were optimized based on impermeability. The fatigue tests were designed with the self-developed loading device, and the variation rules of electric flux under fatigue load were analyzed. The evolution of microstructure was studied to reveal the attenuation mechanism of impermeability. The results show that the impermeability of concrete increases first and then decreases with the rise of basalt fiber length and content. Based on impermeability, the optimal length and content of basalt fiber are 12 mm and 0.08% (mass fraction). With the rise of fatigue times and stress ratio, the increase of electric flux of basalt fiber reinforced concrete is lower than that of reference concrete, which is more suitable for heavy load and heavy traffic areas. Basalt fiber refines the pore structure of concrete, and the proportion of harmless pore increases by 18.51%. The pore size of concrete shows an expansion trend and the pore distribution is coarser under fatigue load. The microcracks mainly extend in the length direction at the early stage of fatigue loading, while the microcracks mainly extend in the width direction at the later stage. Basalt fiber delays the deterioration of pore structure and the propagation of microcracks, so as to reduce the attenuation range of impermeability.