The electric flux tests, scanning electron microscope and mechanical tests were carried out respectively on cementitious composites with single mixing and compound mixing CaCO₃ whisker and PVA fiber to investigate chloride ion resistance of multi-scale fiber reinforced cementitious composites. The effects and mechanisms of different fiber sizes, amount of admixture and mixing ratios on the chloride ion resistance and basic mechanical properties of cementitious composites were analyzed. In addition, the calculation method of the depth of chloride ion erosion was proposed. The results showed that different types of fibers can enhance cementitious composites at different structural levels, and the chloride ion resistance of multi-scale fiber reinforced cementitious composites is much better than that of single fiber reinforced cementitious composites. The compressive strength of composites is inversely proportional to the depth of chloride ion erosion and the electric flux. When the compressive strength of the composite was increased by 13.6%, the depth of chloride ion erosion and the electric flux were decreased by 39.1% and 44.7%, respectively. Based on the experimental data, a calculation model of chloride ion erosion depth considering the influence of compressive strength is established, which can be used to evaluate the chloride ion penetration and erosion resistance of cement-based materials reinforced by multi-fiber composite.