The Karamay oil sand in Xinjiang province, northwest China, forms a large heavy oil reserve. At present, a lack of understanding of the wettability of the oil sands precludes the engineers from properly characterizing the efficiency of fluid injection in either water or steam-flood practices. In this study, a series of experiments were conducted on a specimen obtained from a Karamay oil sand reservoir, including contact angle tests, scanning electron microscopy (SEM) observations, nuclear magnetic resonance (NMR) tests, and zeta-potential tests. On the basis of the experimental results, the wettability of the Karamay oil sand as well as the influences of the differential pressure and the pH on its wettability were analyzed in detail. It was found that there exists a water film on the oil sand grains, implying the oil sand is water-wet. In addition, the presence and stability of the water film were analyzed using the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. Under the reservoir conditions, a water film with a thickness of 2-7 nm forms in the Karamay oil sand. Either increasing the injection pressure difference or enhancing the alkaline environment during water injection will contribute to the formation of a stable water film.