Abstract: A new technique of two times adding quicklime for improving soft-soils was developed and used during Jiangsu expressway construction. It not only settled effectively the problems of soft soil smashed difficulty, gray-doped uneven, and slow construction, and in-effect of quicklime, but also increased the strength of quicklime mixed -soft soils. The test results show that the CBR value of lime-soil mixtures increases by 6% to 8%. This paper examines the mechanism of the technique. At the first time, when it is added to soft soils, the quicklime absorbs the soil moisture and evaporates the soil moisture by the heat of quicklime degradation, and leads to ions exchange. At the second time, when it is further added to the pre-treated soft-soils, the quicklime increases further the volcanic ash and ion exchange, forms new crystals and structure, and subsequently, improves the strength and stability of the lime-soil mixture through the compaction of lime and soft soil.