The tunnel’s cast-in-place inner drainage ditch is a thin-walled structure, and its construction joints are weak points in terms of waterproofing. Water leakage often occurs due to concrete cracking at the construction joints of the drainage ditch and the failure of waterproofing measures. Buried rubber waterstops are commonly used for waterproofing tunnel drainage ditch construction joints. Therefore, it is essential to understand the waterproofing function and mechanical properties of rubber waterstops at tunnel drainage ditch construction joints. Based on the waterproofing mechanism of waterstops and combined with numerical simulation, this paper analyzes the failure range and mechanical characteristics of buried rubber waterstops at construction joints in tunnel drainage ditches under different structural deformations, bonding forces, installation angles, and water pressures. The results indicate that, the shear resistance of the waterstop is superior to its tensile strength; the area of interface bonding failure decreases as the interface bonding strength increases; the stress in the concrete structure increases sharply with increasing angle; Under the simplification that the hydrodynamic pressure is equal to the hydrostatic pressure, the reaction force of the groove decreases gradually as the water pressure increases, and the water pressure is mainly borne by the concrete structure.