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Synthetic fibers can effectively inhibit the formation and propagation of micro-cracks in concrete, significantly reducing the number and scale of cracks within the concrete matrix, thereby enhancing the concrete's crack resistance and seepage prevention capabilities. In this study, two types of synthetic fibers, polyvinyl alcohol (PVA) and polypropylene (PP), were incorporated into cement mortar to investigate their microstructural evolution at elevated temperatures and their influence on the mechanical properties of the mortar. Both fibers were added at a volume content of 0.5%. The mortar samples were subjected to the following temperature conditions: 20 °C (ambient), 200 °C, 400 °C, and 500 °C. The results indicate that the synthetic fibers employed in this study improved the tensile properties of the mortar at room temperature (20 °C). This enhancement persisted up to 400 °C, beyond which, at 500 °C, the mechanical properties of the fiber-reinforced mortar deteriorated significantly. At 400 °C, the tensile strength of the PVA group increased by approximately 16% compared to the unblended fiber group (JZ) and by about 45% compared to the PP group. After treatment at 500 °C, the tensile strength of mortar specimens in the PVA group and the PP group decreased by 36.47% and 24.14%, respectively, compared with that at 20 °C. The porous structure formed due to the high-temperature ablation of the synthetic fibers contributed to relieving the internal pressure within the mortar. [ABSTRACT FROM AUTHOR]