Pretreated coconut fibers can be applied as renewable damping components in fiber-reinforced cement mortars to reduce the adverse effects of vibration and promote the recycling of waste fibers. In this study, FT-IR, TG, XRD, and SEM measurements were performed to analyze the morphologies and physicochemical properties of the coconut fibers before and after pretreatment. The compressive strength, porosity, and damping properties were tested to study the effects of fiber pre-treatment on the mechanical properties of cement mortar. In contrast to original fibers, pretreated coconut fibers induced a moderate air-entraining effect in cement mortars. Therefore, the mortars with pretreated fibers exhibited significantly lower strength loss, indicating consistency with the porosity results. Furthermore, by adding 0.75 vol% coconut fibers pretreated by a mixed solution of NaOH and H₂O₂, the loss tangent of the cement mortar can be increased by 25%. The enhanced damping properties were attributed to the weakened interface between the fibers and cement mortar matrix, which facilitated dissipation of the vibration energy. Moreover, a simplified shear-lag model was proposed to describe the relationship between the weakened interface frictional sliding and the damping properties of the cement mortars.