For strong edge-bottom water heavy oil reservoirs in Pearl River Mouth Basin of the South China Sea, its development methods are based on mostly horizontal wells, large liquid volumes and natural energy. The reservoirs are strongly washed, and most of these reservoirs have now entered high. water-cut period. This development method will inevitably change the wettability of the reservoir, and the wettability is an important factor affecting the development of the oilfield. Therefore, it is necessary to carry out the high-multiple water flooding process research. Thus, the conventional experimental procedures were improved, and high-power water flooding experiments were carried out using underground oil samples and underground cores. The contact angle was measured of different oil viscosity and flooding velocity before and after high-multiple water flooding. Different methods were used to analyze the microscopic mechanism of the change. The experimental results show that the oil viscosity affects the initial wettability of the core and the wettability after high-multiple water flooding, and the effect of high-multiple water flooding on different oil viscosity is different. The flooding velocity changes the wettability of the cores, although the impact is relatively small. The contact angle after core flooding at a higher displacement velocity is slightly smaller than the contact angle at a lower velocity, indicating that increasing the displacement velocity can make the core more water-wet to a certain extent.