Abstract: This paper shows that the "nonhydrocarbon+bitumen" is the key matter which affects the surface wettability of an oil reservoir; When its content arrives at a certain value, the surface wettability of narrow constrictions in the pores of the reservoir may convert into oil-wet. However, when the surface of pore throat is oil-wet, the determination of the wettability of rock sample is dependent upon the amount of the water saturation. It is oil-wet when the water saturation is below 30%, and water-wet when it is over 40%. It is shown that the wettability of the rock is gradually transformed from oil-wet to water-wet as the water injection is goiog on.