To study the anti-permeability mechanism of polypropylene fiber reinforced concrete (PFRC) under preloading，one type of coarse polypropylene fiber and two types of fine polypropylene fibers were selected to add into concrete. The diffusion coefficient of chloride ion of hybrid PFRC under five different preloading levels was analyzed by chloride diffusion coefficient test. The mercury intrusion porosimetry (MIP) and the scanning electron microscopy (SEM) were used to investigate the effect of fiber hybridization and preloading level on the micropore structure and permeability of concrete. The test results showed that the chloride diffusion coefficient of PFRC specimen A8 under five preloading levels decreased by 93.9%，90.8%，77.5%，63.5% and 66.5%，respectively，all of which have the largest reduction，indicating that the anti-chloride ion permeability of hybrid PFRC was superior to plain concrete and the concrete reinforced with one kind of fiber. Moreover，multi-scale fiber hybridization with coarse and fine fibers can form a three-dimensional spatial network structure，resulting in a synergistic effect at different levels，and the anti-permeability of concrete under different loading levels was effectively improved.