The invert arch floor of large plunge pool is composed of some discrete arch blocks divided by construction joints. The joint is only 1 mm wide. Under the uplift pressure, the mechanical behavior belongs to highly nonlinear problem, which is difficult to simulate by finite element method. Based on NMM (numerical manifold method), the kinematic motion process of the invert arch floor is simulated under the different uplift pressure. Calculation results show that: as the uplift pressure increasing, each arch block gradually rises, rotates, contacts with adjacent blocks and the expansion joint width between adjacent blocks reduces. When uplift pressure reaches 85Kpa, the mutation of expansion joint width emerges and the invert arch floor finishes self-lock process which indicating the invert arch floor start playing statically indeterminate role to keep stable. The bearing capacity of the floor is determined by concrete compressive strength and anchoring strength of two arch abutments.