In order to solve the disease problem of Suida expressway, the loess-like sub-sandy soil was modified by the addition of new materials lithium silicate, lime and polypropylene fiber. The strength characteristic, microscopic mechanism, curing mechanism and pore structure of the modified soil were studied by the freeze-thaw cycle test, X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that the addition of lithium silicate can greatly improve the unconfined compressive strength of lime modified soil. The optimal mix ratio of composite modified soil is that: the content of lithium silicate is 3% (mass fraction), the content of lime is 6% (mass fraction), the content of polypropylene fiber is 0.4% (mass fraction) and the fiber length is 12 mm. The destructive form of composite modified soil is "ductile failure", and the polypropylene fiber plays a pulling role in its stress process, so that the specimen can withstand greater stress within a certain strain range. Regardless of whether the modified soil is in a standard conservation environment or a freeze-thaw cycle environment, lithium silicate can play the role of soil strength enhancer, provide an alkaline environment, intensify the hydration reaction, and generate new substances such as albite, C-A-S-H gel, Li［AlSi4O10］and so on. The formation of hydration products is the main reason for the decline of soil porosity related parameters. The standard conservation environment and alkaline environment can promote the hydration reaction, improve the decline of pore parameters, and effectively improve the strength of modified soil.