It is unreasonable to calculate the buoyancy force value of underground structures near riverside directly using ground elevation or river level as anti-buoyancy water level. Seepage analysis of the field is necessary for anti-buoyancy design. Based on Bennett hypothesis, a simplified algorithm for seepage analysis of double strata near riverside is presented to calculate the hydraulic head distribution in high-permeable layer. A practical method is followed to calculate the buoyancy force on the bottom slab of underground structures for anti-buoyancy design. This method can be applied for different boundary conditions, e.g., the blankets with different thicknesses and permeability on the upstream and downstream of levee , or the blanket with finite or infinite width. This algorithm only requires simple calculation parameters, but the results are accurate enough for using. After verification and comparison, a conclusion can be drawn that downstream small-scale structures in high-permeable layer have little influence on hydraulic head distribution, but the large-scale structures or the narrow-width blanket downstream will rise the head dramatically. The buoyancy in the middle of bottom slab is greater than the surrounding area when the bottom slab of basement locates in the blanket. But it changes to linear distribution when the bottom slab of basement intrudes into high-permeable layer.