Abstract: To clarify the degradation behavior of the tensile properties of bamboo fibers under alkaline solution erosion, four-year-old moso bamboo from Enshi, Hubei Province, China, was used as the raw material. Water absorption tests, alkaline solution erosion tests at different pH values, and tensile mechanical tests were conducted to systematically analyze the water absorption characteristics, mechanical properties, and performance evolution of bamboo fibers under alkaline solution exposure. The results showed that the saturated water absorption of the bamboo fibers reached 112.3%. Their tensile strength and elastic modulus were 1.48 ± 0.39 GPa and 126.3 ± 36.86 GPa, respectively, and both were related to fiber diameter. Alkaline erosion markedly weakened the mechanical properties of the bamboo fibers. After immersion in a pH 12 alkaline solution for 21 days, the tensile strength and elastic modulus decreased by 45.9% and 43.3%, respectively. Under the pH 13 condition, the strength loss rate was more than 40% higher than that under the pH 11 condition. With increasing erosion duration, the mechanical properties showed a staged degradation pattern, with rapid deterioration during 0–21 days and slower degradation during 21–45 days. The erosion mechanism was attributed to water transport, which promoted the diffusion of OH⁻ ions into the cell wall. This process led to the dissolution of lignin and hemicellulose, damage to the cellulose structure, reconstruction of the pore structure, and expansion of microcracks. These findings provide a basis for service-life prediction and durability design of bamboo fiber composites used in alkaline environments.