Sandstone core and free-clay sandstone core were prepared in chromatography columns, and core flooding experiments were performed to determine the EOR mechanisms of low salinity (LS) water flooding and analyze the effect of clay on recovery factor. First, brine with a high salinity (HS) of 100 000 mg/L was injected to simulate formation water. Then, LS water of 1 000 mg/L was then injected at different temperatures. Concentrations of Ca ²⁺ and CH₃COO ^- and pH of the fluid flowing out of the chromatography column were recorded. The results showed that during flooding the free-clay core with LS water, the carboxylic acid detached from the sand, as great as that of the clay-containing cores. It showed a noteworthy desorption of Ca ²⁺ from the free-clay core surface, which indicated that LS water flooding resulted in a high recovery in sandstone without clay. Compared with HS water flooding, in the LS water flooding, the pH of produced fluid increased significantly; also, as the temperature increased, the pH and Ca ²⁺ concentration of the produced fluid decreased; and the hydrophilic property of the sandstone was stronger. The results of oil recovery tests using the same columns were in line with the chemical analyses. The oil recovery from the free-clay columns was as great as from the column clay-containing column. The results of wettability measurements supported our theory.