Removing trace water from oil is an important industrial process and is commonly accomplished using vacuum filtration. The drawbacks of this method, however, are: poor efficiency, large oil loss, and significant energy consumption. The objective of the current study was to develop a better system to solve these problems using a sodium polyacrylate (PAA-Na)-modified bentonite as the dehydrating agent and, for the first time, to apply it to transformer oil. PAA-Na was prepared by aqueous solution polymerization. A dehydration test was carried out to determine the optimum addition of PAA-Na, and the highest dehydration rate of 76.5% was obtained with the addition of 20 wt.% PAA-Na. The steady dehydration rate of the PAA-Na-modified bentonite was better than that of other adsorbents (calcium chloride, zeolite 5A, unmodified bentonite). The process of adsorbing saturated water vapor on PAA-Na modified bentonite was studied and interpreted from the aspects of adsorption isotherms and thermodynamic properties. The results showed that the adsorption isotherm data followed the Freundlich isotherm model and the thermodynamic parameters indicated that the process was endothermic. Fourier-transform infrared spectroscopy results revealed that PAA-Na was synthesized successfully and it had a huge proportion of hydrophilic groups. According to thermogravimetric analysis, the PAA-Na-modified bentonite was stable up to 200°C, giving a flexible region for pretreatment and regeneration. X-ray diffraction showed no change in the diffraction pattern before and after modification. Moreover, considering the results of scanning electron microscopy and surface-area analyses, one may safely say that PAA-Na was distributed homogeneously on the surface of the bentonite. In addition, PAA-Na-modified bentonite exhibited a high dehydration rate in xylene, naphtha, and diesel, indicating a broad range of applications.