The red-bed residual soil has the characteristics of high liquid limit, low strength and strong expansion and contraction, which cannot be directly used as filler in the project. Through a variety of modified materials pre-mixing, high-efficiency materials re-mixing, and finally proposed to use quicklime-ash wood fiber as an external mixer to improve it. By carrying out shear test, CBR (California bearing ratio) penetration test, mercury pressure test and scanning electron microscope test on specimens with different dosage, the improvement mechanism of quicklime-ash lignocellulosic fibers on the macro-mechanics and micro-porosity of the soil was analyzed. The results show as follaos. The shear strength and cohesive strength of the modified soil increase and then decrease with the increase of lime dosage, and the optimal dosage of modified soil is 4% ash wood fiber + 4% quicklime. The pore size distribution shows that the proportion of large pores and micropores in the modified soil increases, the medium pores decrease, and the proportion of other pores fluctuates steadily. Based on the electron microscope scanning analysis by PCAS software, the pore space and interlayer fissures of the improved soil are reduced compared with the vegetative soil, and the inter-agglomerate cementation capacity is enhanced and the structure is stabilized.