In order to investigate the effect of naphthalene sulfonate water reducer (FDN-C) used in concrete on soil solidification, different amounts of FDN-C were added into high plastic clay, compared with calcium lignosulfonate (CA-LS), the compaction load, unconfined compressive strength (UCS) were tested. The mechanism was analyzed by testing the particle size, microstructure, bonded water, mineral composition and functional group of solidification treated samples. The results show that, like CA-LS, FDN-C can also reduce the compaction load and improve the UCS. When the dosage is higher than 0.7% of soil sample mass, the UCS has little difference, while the compaction load of samples added FDN-C is lower. FDN-C and CA-LS improve the UCS by dispersing soil particles, reducing the thickness of bonded water on the surface of soil particles, and reducing the particle spacing, so that the soil particles can be better filled and compacted with each other, thus improving the engineering properties of soil. CA-LS has a strong ability to reduce the thickness of weak bonded water, while FDN-C reduces the thickness of strong bonded water. More free vacancies in CA-LS molecules make it have certain adhesion, which can make soil particles better cemented together, but the similar effect is not found with FDN-C.