The equilibrium between gravity of the upper impermeable stratum and water pressure is a common calculation method for anti-uprush of foundation pit. In case the effective unit weight of upper stratum is less than the seepage force,its low strength and high permeability will lead to the instability of the bottom of the foundation pit,and the calculation method equilibrating between effective unit weight and seepage force should be adopted in this case. The two kinds of the calculation methods are confusable in practical application. Similarities and differences between the methods,such as the expression,principium,safety coefficient and application,are analyzed,and the principles to choose the methods are discussed. The methods are also applied to a engineering project,and the achieved results indicate that the reasonable choice of calculation methods of anti-floating for foundation pit is important,and some valuable conclusions are drawn.