Abstract: Currently, there are few studies on the multi-phase flow law of high-pour-point oil under high pressure. Focusing on the phenomena of the relative permeability curve fluctuation and wettability hysteresis in the water-flooding process of high-pour-point oil, the effect of solid particles in different periods was described microscopically; moreover, three hypotheses on wettability reversal was also proposed, including in-situ growth, water removal and oil in, and film coating. The results verified by experiments and numerical simulation indicate that the plugging and migration of solid particles at pore throat leads to the fluctuation of the displacement pressure difference and the waterflood front advance velocity, which is also the primary cause for the oscillation of relative permeability curve; the water film on the pore wall will inhibit the transformation of core surface to oil wet surface; the wax deposit thickness on the water film is smaller than that of rock wall, and the difference between them can reach 21.5%; the water film heterogeneity results in that the wax deposit thickness in the core is only 59%-77% of that of dry core (without connate water), and the transformation of core wettability to oil wet is accelerated as the water film thickness decreases; low-permeability cores are more easily affected by temperature than high-permeability cores, and the oil recovery of low-permeability cores is the lowest, which is only 23.6% of that of high-permeability cores.