To help solve the soil salinization problem of the dam downstream area caused by reservoir leakage in plain arid areas, we studied the buried depth of the farmland underground water level in the downstream area of the Qiala Reservoir dam based on the unsaturated seepage theory, as well as the influence of“prevention-interception-diversion” seepage control method on the buried depth using numerical simulation. The results showed that the change of the depth and the water collecting depth of the interception ditch could affect the groundwater buried depth of the farmland in the downstream area of the dam. Under the combined action of “prevention-interception-diversion”, the optimal depths of the interception ditches were determined to be 4.5 m, 4.5 m and 3.5 m, respectively under the three working conditions of no seepage control measures, horizontal bedding and vertical cut-off wall. Among them, the effect of the vertical cut-off wall combined with “interception-diversion” method performed the best, which was 1.16-1.30 times of no seepage control measures, and 1.15-1.21 times of horizontal bedding measures. It can be seen that the comprehensive seepage control mechanism of “prevention-interception-diversion” can effectively control the seepage flow in the downstream so as to reduce the farmland underground water level, prevent the farmland soil salinization, and control the downstream ecological environment.