The invention relates to a detachable arch dam and construction and detaching methods thereof and belongs to the technical field of water conservancy projects. The detachable arch dam comprises a drainage grouting gallery, an anti-seepage curtain, dam foundation drainage holes and a cross joint, and further comprises an upstream concrete panel, building block concrete, a downstream concrete panel,a plain concrete cushion layer, dam foundation anchor bars, a dam top concrete plate and an arch support concrete cushion layer. The reinforced concrete panels are poured on the upstream side and thedownstream side of a building block dam body. The faces, facing the upstream direction and the downstream direction, of the building block concrete are arc faces, and the adopted arc radius is the arc radius of the upstream face and the downstream face of an arch ring. Concrete building blocks can be fully utilized, industrial production is facilitated, the mass is easy to control, temperature cracks are avoided, the temperature control requirements in the pouring process of the dam body are reduced, and safety of the dam body is better ensured; the building blocks are easy and convenient tobuild, the construction efficiency is high, the influence of weather is small, and the construction speed of the dam body can be greatly increased; and the building blocks are convenient to detach, detached prefabricated parts can be recycled and reused, and economical efficiency and environment friendliness are both achieved.