In order to solve the problem of longitudinal cracks in the high liquid limit clay embankment in the Bengbu section of the Huaihe River Basin, the high liquid limit clay was modified by using cement and fibers. Through the research on the plane strain tests of the modified high liquid limit clay with different contents of cement and fibers, the variation relationships of the stress-strain curve, the intermediate principal stress and the elastic modulus of the modified soil with the contents of cement and fibers were obtained. Combined with the microstructure test, the mechanism of the modification of high liquid limit clay by cement and fibers was revealed. The results show under the plane strain state, the addition of cement and fibers significantly improve the strength of high liquid limit clay. For the modified high liquid limit clay, the influence of the intermediate principle stress on the destruction of the soil gradually decreases as the cement content increases. Under low cement content conditions, the stress-strain curve shows hardening type or weak softening type, the brittleness index of the sample is low, and no obvious shear band is observed during sample failure, the influence of fiber content on the failure morphology of the sample is low. When the cement content is high, the stress-strain curve shows a strong softening type, the brittleness index of the sample decreases as the fiber content increases, and obvious shear band is observed during sample failure, which is weakened gradually as the fiber content increases. From the microstructure analysis, cement improves the effective stress transmission between soil particles, while fibers disrupt the effective stress transmission, and the interaction between the two causes the trace of the shear band and the form of the sliding plane to change.