Submarine landslides are usually enormous in size but often developed from a minute slip surface. Attention has previously been paid to quantifying the failure initiation of a submarine landslide through two-dimensional (2D) plane strain slope stability analyses. The findings of failure initiation from the 2D simplifications need to be justified in a realistic 3D scenario, and more importantly, are inconvenient to apply into analysing the subsequent 3D post-failure behaviours. This study aims to explore to discover the true physical mechanism of submarine landslides and to establish practical criteria for submarine slope stability analysis, by modelling and investigating the whole 3D landslide evolution integrating both the failure initiation and post-failure behaviours. The numerical method is formulated by solving governing equations in terms of the conservations of mass and momentum considering isotropic and linear strain softening materials. Ability of this framework to simulate a complete landslide evolution, including the initiation and growth of slip surface, global slab failure, post-failure behaviours and re-deposition, has been demonstrated for different slope geometries. The proposed numerical scheme is able to capture diverse post-failure behaviours, such as retrogression and blocky slide mass, in sensitive soils. The characteristics of the slip surface growth within a favoured layer and the patterns of the global slab failure in the overlying layer have been thoroughly discussed. For planar slopes, it helps to establish an analytical criterion for unstable dynamic growth of a planar slip surface, which can optimise the slope stability analysis in sensitive soils.