A contrast study on effect of powdered, aminated straw with or without inorganic soil conditioner（calcium sulfate）on soil structure and water-holding capacity in low suction section was made through soil column incubation test. The results showed that the pretreated straw significantly improved soil structure stability, decreased soil structure fractal dimension. Therein the soil with comminuted and aminated straw had a slow acting to improve soil stability. Different pretreatment of straw mixed with soil could make soil porosity connectivity poor in the early stages, and then better with the straw decomposition later. Therein the soil with long straw always kept worse soil porosity than soils with the powdered straw. Furthermore there was a significantly linear correlation between soil aggregate fractal dimension and soil porosity fractal dimension in low suction section. That quantitively explained the effect of the pretreated straw on soil structure and water-holding capacity in low suction section of soil.