Due to the negative impact of abandoned powdered clay on land waste and pollution, it is beneficial to improve the powdered clay and use it for backfill in engineering construction. The effects of lignin fiber content and cement content on the unconfined compressive strength of silty clay excavated from a tunnel along the Yangtze River in Hangzhou, Zhejiang Province were studied. The formation mechanism of the compressive strength was analyzed. Finally, the pore microstructure of the sample was quantitatively analyzed by SEM experiment. The results show that the compressive strength reached the maximum value when the lignin fiber content is 4%, and the unconfined compressive strength was greater than that when the lignin fiber content is 2%, 6% and 8%. No matter how much lignin fiber content is, the unconfined compressive strength increases gradually with the increase of cement content. With the addition of lignin fiber, the average diameter of pores gradually concentrated in the range of particle size less than 1 μm, and the proportion of pores 1 ~2 μm and 2 ~4 μm increased with the increase of cement content. With the increase of lignin fiber and cement content, pore abundance mainly concentrated in the range of 0.2 ~ 0.5. With the addition of lignin fiber, the particle abundance mainly concentrated in the range of 0.1 ~ 0.6, and the particle distribution showed a "mountain" pattern with the addition of cement.