The core wall of earth-rockfill dam is considered as a vertical cantilever beam, the upstream side of which is controlled by the pockety earth pressure of the upstream shell and the time-varying water pressure, and the downstream side is also controlled by the corresponding earth pressure of the downstream shell and the water pressure similarly. When the reservoir water level rises and the core wall has a tendency to turn to the downstream side, the upstream earth pressure gradually decreases and the downstream earth pressure gradually increases. Once the earth pressure on both sides reaches the limit active and passive earth pressure respectively, slippage is likely to take place, and the core wall deflection at this time is defined as the safe deflection f_s. If the measured actual lateral displacement of the core wall is larger than f_s, we can roughly determine that relative sliding of the soil on both sides of core wall exists, which may generate local longitudinal crack. The research result can be applied directly to the calculation of horizontal deformation and safety assessment for concrete core wall of similar conditions.