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Superabsorbent polymers (SAPs) are type of hydrogels capable to swell and absorb a large amount of water, but easily decomposed and oxidized by the air. We used electron-microscopic imaging in an indoor simulation with sand mulching to test the effects of various SAP concentrations on controlling evaporation and salt formation. The treatments were sand-mulched columns containing 0, 0.1, 0.2, 0.5 and 1.0% SAP. The soil particle pores were from dense to sparse and the corresponding fractal dimension decreased as SAP concentration increased. SAP concentration was correlated negatively with fractal dimension, clay-particle fraction and silt-volume fraction. And it showed a positive correlation with sand volume fraction. SAP concentration significantly affected the particle-size distribution. Water-storage capacity increased in each column layer (five 8-cm layers) at the same infiltration depth. Evaporation decreased the water content of each layer. Sand mulching combined with the SAP decreased evaporation in each layer relative to the control, which retained more water and decreased the accumulation of surface salt in the order 1.0% > 0.5% > 0.2% > 0.1% > 0. Salt migrated at 0–30 cm with sand mulching but 0–25 cm with sand mulching and SAP amendment. The decrease in salt accumulation was most effective at a SAP concentration of 0.2%.