Abstract: For the gravelly sand type sulfate soil with sodium sulfate content of 2%, the compaction tests, salt expansion tests, collapsibility tests and unconfined compressive strength tests with different ratios of lime, lime + volcanic ash are carried out. Based on these tests, the improved mechanism and micro-characteristics are analyzed. The results show that adding lime or lime + volcanic ash can reduce the salt expansion effectively and reduce the sensitive temperature range of salt expansion. In the case of using inorganic modifiers to improve the salt expansion deformation of the sulfate saline soil, considering the local climatic conditions, the amount of modifiers can be appropriately reduced in the areas with higher temperature, and when the temperature is lower, the amount of modifiers should be appropriately increased. Compared to that of the lime-improved soil, the compaction stage of the volcanic ash-improved soil is shortened, the elastic stage is increased, the generated strain when the soil sample reaches the strength limit is reduced, the soil structure is stronger, and the deformation resistance is enhanced. The addition of volcanic ash can also accelerate the strength growth rate of such saline soils. The deformation rate of salt expansion and collapsibility are both less than 1% by adding lime higher than 11% or lime +volcanic ash not less than 15%, and the 7-day saturated unconfined compressive strength of the improved soil is not less than 0.35 MPa.