Abstract: In this paper, the mechanical properties of silty clay reinforced with cement and polypropylene fibers were studied through direct shear tests. Additionally, water immersion softening tests were conducted to simulate the effects of long-term rainfall on the strength of both plain and reinforced soils, revealing the mechanical behaviour and microstructural changes of the samples after softening. The results indicate that the cohesion and internal friction angle of the reinforced soil increase with higher cement and fiber content, but the rate of increase diminishes behind a certain threshold. Based on these findings, the optimal reinforcement composition is determined to be 6% cement and 0.4% polypropylene fibers. Water immersion softening tests conducted on samples with this composition reveal the following characteristics: In the initial stage of water immersion, due to the hydration and hydrolysis reaction, the cementation between soil particles is enhanced, and the close bonding between soil and fibers further increases the cohesion of the soil. Energy exchange between soil and water ions in the soil reduces the thickness of the water film bound to clay particles, slightly increasing the internal friction angle; As the immersion time increases, the accumulation of free water leads to relative movement and dispersion of soil particles into blocks. However, the strong cementation provided by hydration products and the encapsulating effect of fibers maintain the soil's structural integrity, resulting in a gradual decrease in cohesion and internal friction angle. These findings provide critical mechanical parameters and theoretical insights for the application of cement and polypropylene fiber reinforcement in cohesive soils.