Abstract: Soil conditioners, which are widely used in preventing soil and water losses, could improve soil physical structure and provide good conditions for plant growth. PAM, water super absorbent polymer，βcyclodextrin and humic acid were used as soil conditioners in this paper. The results showed that all of the four conditioners could promote the formation of >0.25mm waterstable aggregates. Within the test range of 0.05%-0.40%, the improved effect showed PAM>βcyclodextrin >water super absorbent polymer > humic acid, and the fractal dimension of aggregates decreased largely, especially for the PAM. All of the four conditioners could reduce the soil bulk density at the low concentration. From soil particle size distribution, it can be observed that 1mm soil waterstable aggregates, especially >5mm soil waterstable aggregates obviously, which will explain how PAM worked on soil structure. In the four conditioners, PAM played the best role in improving loessal soil structure. This paper discovered the effect and mechanism of soil conditioners on loessal soil, which will provide theoretic basis of using and improving soil conditioners reasonably.