The unconfined compressive strength (UCS) test was carried out on the waste silty clay from a tunnel excavation in Hangzhou City after adding different amendments. The compressive strength characteristics of the improved clay sample under different slag content, slag to fly ash ratio and polypropylene fiber content were studied. The formation mechanism of compressive strength was briefly analyzed. At the same time, based on scanning electron microscope (SEM) test, the pore characteristics of the reaction soil were quantitatively analyzed. The results show that the maximum UCS value is 6 738.04 kPa when the slag content is 12%, which is 80.69 times higher than that of plain soil. The addition of fly ash can improve the late strength of the sample, which is the best in K8F2 (slag and fly ash ratio of 80% ∶ 20%) sample. The addition of polypropylene fiber can effectively improve the brittleness of the material, and the optimal fiber content is 0.4%. The orientation frequency of the pores of the sample is the largest distribution probability in the range of 80° ~ 100°, and the orientation is more uniform after the addition of fly ash. With the increase of slag content and the increase of slag content in geopolymer, the porosity decreases gradually, and the minimum value appeares in K12 (slag content 12%) and K8F2 samples. With the increase of fiber content, the value of fiber content decreases first and then increases, and the minimum value is 6.204% when the fiber content is 0.4%. The average pore diameter is mainly 0 ~ 1 μm, and the small pores increase and the large pores decrease with the increase of slag content. The pore abundance is mainly distributed in the range of 0.4 ~ 0.6, and the pore shape is mainly elliptical.