Abstract: The Longyou Grottoes are a series of ultra-large underground constructions carved by hand in ancient China, whose roof rock mass stability is threatened due to the low strength of the argillaceous siltstone and the weak layer structures. In particular, variations in the thickness and dip angle of weak interlayers have significant effects on the bending strength and failure modes of the roof. However, existing studies on the stability of the roof in the Longyou Grottoes are limited and have overlooked the mechanical response of the roof to the entire weak interlayer. In this paper, a model with alternating soft and hard layers, varying in dip angle, thickness, sequence, and mechanical properties, was established using the discrete element method with Particle Flow Code (PFC). This study systematically examines the effect of weak mudstone interlayers on the three-point bending characteristics of the roof's argillaceous siltstone. The simulation results show that the degradation of the weak interlayer's strength leads to a general decline in the overall bending strength of the roof rock. Under low dip angles, the sequence of layers significantly affects the bending strength, with the hard layer at the lower boundary enhancing it, and cracks tend to develop across layers. As the dip angle increases, the effect of the layer sequence diminishes, and cracks tend to develop along the layers. Additionally, the combined influence of the layer sequence and dip angle controls the impact of layer thickness on the bending performance. However, this control weakens as the layer thickness decreases, leading to a stabilization in bending performance. Within the dip angle range of the Longyou Grottoes' layers, the strength degradation of weak mudstone interlayers leads to a sharp decline in bending performance, making the roof prone to failure along the weak layers. This study enhances the understanding of the mechanical behavior of layered roofs and provides important references for the stability evaluation and protective strategies of the Longyou Grottoes' roof.