Currently, the thickness design method for ecological soil covers used in Northwest China is based on the practice of non-humid regions in North America, which is complicated, involves many parameters, and fails to consider the different meteorological conditions between Northwest China and North America. To solve this issue, on the basis of the monsoon climate and the feature that the precipitation during the non-growing period of vegetation shall not exceed 20% of the total annual precipitation in Northwest China, as well as the water retention characteristics of ideal cover soils that the ratio of the field capacity to the wilting point K ≥ 1.44, the thickness design method for ecological soil covers used in Northwest China is simplified. Numerical program Vadose/W is utilized to analyze the hydraulic responses of the loess monolithic cover and the loess-gravel capillary barrier cover designed with the proposed simplified method in the year of maximum precipitation. The results indicate that the capillary barrier effect significantly prevents the migration of water from the loess layer to the gravel layer, resulting in the capillary barrier cover having better anti-seepage performance than the monolithic cover. The anti-seepage performance of the two covers decreases with the decrease in the ratio of the field capacity to the wilting point of the loess (i.e., K). When K ≥ 1.35, the two covers can meet the local anti-seepage requirements, thus preliminarily verifying the reliability of the simplified method proposed in this study. The proposal of the simplified method is conducive to the scientific design and application of ecological soil covers in Northwest China.