Abstract: To investigate the damage characteristics of concrete arch slabs subjected to underwater contact explosions, a series of underwater contact explosion tests were conducted on three concrete arch slabs. During the experiments, the spalled fragments generated after detonation were carefully collected and quantitatively analyzed using a mass-based partitioning statistical method, enabling a systematic characterization of the fragmentation and spalling behavior of the slabs. On this basis, a three-dimensional fluid–structure interaction numerical model incorporating water, a concrete arch slab, and TNT was developed using the Coupled Eulerian-Lagrangian (CEL) method. The reliability and effectiveness of the numerical model were validated by comparing the simulated macroscopic failure patterns and crack propagation paths with the experimental observations. Based on the validated model, further analyses were carried out to investigate the damage evolution process and stress wave propagation characteristics within the arch slabs under explosive loading, thereby systematically elucidating the damage mechanisms of concrete arch slabs subjected to underwater contact explosions. In addition, parametric studies were performed to examine the effects of explosive charge and span length on the damage characteristics of the arch slabs. The results indicate that, under underwater contact explosion, an annular damage zone composed of radial and circumferential cracks forms on the blast-facing surface of the arch slab. With increasing explosive charge, the number of spalled fragments increases and their size becomes progressively finer. As the span length increases, the structure tends to generate larger fragments. When the TNT charge increases from 0.4 g to 1.6 g, the annular damage zone on the blast-facing surface expands from a localized region to the full width of the slab, and the failure mode transitions from localized damage to global failure. With increasing span length, the damage degree at the arch ends decreases, while the damaged region near the mid-span expands. Compared with the slab with a span of 400 mm, the total energy dissipation of the slab with a span of 600 mm increases by 4.3%.