A proper degree of compactness is very important for reducing the dilatancy and collapsibility of sulfate saline soil foundation. To investigate the compaction characteristics, the microstructure characteristics of sodium sulfate saline soil with different preparation conditions were investigated using scanning electron microscope (SEM). It is found that the compaction characteristics and microstructure characteristics of sodium sulfate saline soil after compaction are related to the state of sodium sulfate in soil. As sodium sulfate solution possesses lubrication, the compaction characteristic of sodium sulfate saline soil is significantly improved as salt content increases. After the soil is compacted, the soil aggregates are mostly thin layers with smooth surface, and the soil has a rhyotaxitic mosaic structure. However, sodium sulfate decahydrate acts like a scaffold in the soil, the increase of salt content reduces significantly the compaction characteristics of sodium sulfate saline soil. After compaction, the soil aggregates are tiny and round, and the soil has a net-like open structure. The effect of sample pretreatment method and curing time on the compactness of sodium sulfate saline soil are analyzed, and the evaluation method for the compactness is suggested.