After waterflooding treatments over several decades, the petrophysical properties of the producing reservoir can change in many aspects. Because the hydrocarbon potential of certain reservoirs is still promising, it is crucial to identify the variation of these petrophysical properties after the long-term waterflooding treatments. Therefore, several groups of core samples, which came from an old oilfield in the northeastern region of China, were collected to investigate the variation in these properties in the reservoir. These samples were drilled and extracted from four different types of inspection wells and thus can represent the different stages of the reservoir during the development history of nearly forty years. A series of tests and experiments were conducted on the samples in order to measure properties such as porosity, permeability, three types of sensitivity, wettability, and internal pore structure. Moreover, the data of experimental results were classified and analyzed using statistical methods. The multiple linear regression model was applied with the predictors such as porosity and sensitivity index. The results showed that certain variables can predict the permeability. Furthermore, both the mean values and standard deviations of the porosity and permeability increased after the long-term waterflooding treatment. The variation of sensitivity, wettability, and pore structure all presented significant directionality. Moreover, the CT scanning experiment directly showed the forming process of large pore channel. Improved understanding of the reservoir properties after long-term-waterflooding treatments will provide valuable guidance for the follow-up production design.