When fibers are uniformly and disorderly dispersed in the cement mortar matrix, subject to external force or internal stress changes, fibers have limited and hindered the extension of micro-cracks to a certain extent. Hundreds of millions of fibers, crisscrossed and isotropic, are evenly distributed in the cement mortar matrix, so that the extension of micro-cracks encounters many obstacles from these fibers, unable to cross these fibers for progress, but only able to detour along the interface between fibers and cement matrix. However, cracking needs energy, so it must break the siege of fibers to crack. The energy produced by stress alone is negligible, which could only be depleted by these fibers. Therefore, due to the existence of fibers in large quantities, it has not only consumed energy, but also eased the stress and prevented further development of cracks, thus playing the role of retarding cracks.