The subducting slab morphology in the mantle transition zone (MTZ) is strongly affected by the mantle viscosity and density variations at the 660-km discontinuity (D660). Besides the negative Clapeyron slope of phase transition and the viscosity increase, a possible thin weak layer at around D660 is proposed to play a key role in the slab stagnation, which is however not well constrained. In this study, a series of numerical models are systematically conducted, which reveal that a weak layer beneath D660 does not change the slab mode selection (penetration versus stagnation). However, it will contribute to longer slab flattening at the bottom of the MTZ, when slab sinking is strongly resisted by either the viscosity increase or a large Clapeyron slope at D660. The role of a weak layer on slab flattening is dependent on the lubrication effect that promotes sub-horizontal slab movement at the bottom of the MTZ.