In this paper, centrifuge tests that are conducted to investigate the mechanism of trapdoor in a sand ground are simulated with 3-D finite element analyses. In the centrifuge tests, a circle shaped trapdoor is adopted and different ratios of the overburden to the width of the trapdoor are considered. The mechanical behaviors of the trapdoor, such as the load-displacement relationship, the distribution of the stresses on the trapdoor and the deformation pattern of ground are carefully investigated. The 3-D finite element analyses are based on a strain softening constitutive model, which satisfies the uniqueness of the solution in an initial value problem on Valanis's sense (Valanis, 1985). It is found that the numerical analyses conducted in this paper can simulate the mechanism of trapdoor to a reasonable accuracy.