Abstract: The unconfined compressive strength properties of lignin modified soil were investigated by unconfined compressive strength tests and scanning electron microscope tests, and the effects of lignin content, curing age, porosity and skeleton void ratio were analyzed. The results show that with the increasing of lignin content, the unconfined compressive strength of modified soil is increased with latter decreasing. The unconfined compressive strength of the modified soil is increased with the increasing of curing age. The established hyperbolic model can predict the relationship among unconfined compressive strength, curing age and lignin content. The addition of lignin changes the porosity and skeleton porosity ratio of the silt. The unconfined compressive strength of the modified soil is decreased with the increasing of skeleton porosity ratio. When the lignin content is 8%, the porosity and skeleton porosity ratio reach the minimum values, while the unconfined compressive strength reaches the maximum value.