Abstract: In soft soil areas,there are two problems in checking the stability against basal heave using Prandtl foundation bearing capacity calculation formula. First,the formula can't reflect that the stability against basal heave increases with the increase of enclosure insertion depth; Secondly,it can't reflect the influence of foundation pit width on the safety factor of anti-uplift stability at the bottom of the wall. The analysis of the difference of mechanical boundary conditions between excavation and the bearing capacity by Prandtl formula indicated that the existing Prandtl formula is not enough to reflect the uplift failure sliding mode of foundation pit. According to the principle of limit equilibrium calculation,this paper proposes an improved calculation model considering the shear action of soil on both sides of foundation pit above the bottom of enclosure wall. The wide and narrow foundation pits are divided under the standard ratio 3 of foundation pit width-depth. The formulas for calculating the stability against basal heave of the wide and narrow foundation pits are given respectively. The calculation of engineering examples shows that the improved formula is consistent with the safety situation of successful engineering examples. It explains the law that the stability of foundation pit against basal heave is improved with the increase of enclosure insertion ratio in soft soil areas. According to the division of wide and narrow foundation pits,the safety factor of against basal heave stability of narrow foundation pits can be calculated reasonably. The research results of this paper can contribute to the calculation and evaluation of stability of foundation pit against basal heave in soft soil area,and to the structure optimization design of foundation pit. It also needs to be supplemented and improved by further theoretical research and verified by engineering practice.