Based on slip field theory, analyses the stress state of passive slip zone BDE, transition zone BCD and active slip zone ABC at the excavation bottom, and infers the slip-line solution to soil ultimate bearing capacity of the excavation bottom at the assumed supporting depths t, and calculates the supporting depth in accordance with the soft soil excavation against heave stability safety requirements. Moreover determines the safety coefficient against heave stability for anti heave stability checking, and under homogeneous soil conditions, through the simple conversion obtains the same result with the safety coefficient k of excavation against heave stability by Prandtl limit equilibrium theory. Combined with engineering examples, slip-line solution of supporting depth in soft soil excavations and soft soil foundation heave stability safety coefficient are verified to be reasonable.