A semiconductor structure and a forming method thereof are provided. In one form, a semiconductor structure includes: a substrate; discrete channel structures on the substrate in device regions; a power rail line, located in the substrate of a power rail region; a gate structure, extending across the channel structures; source/drain doped regions, located in the channel structures on two sides of the gate structure; an interlayer dielectric layer, located at a side portion of the gate structure; a power rail contact plug, penetrating a partial thickness of the interlayer dielectric layer at a top of the power rail line, where the power rail contact plug is in full contact with a top surface of the power rail line in a longitudinal direction; and a source/drain contact layer, located in the interlayer dielectric layer and in contact with the source/drain doped region, where on a projection surface parallel to the substrate, the source/drain contact layer extends across the power rail line. The power rail contact plug is in full contact with the top surface of the power rail line in the longitudinal direction, and a dimension of the power rail contact plug in the longitudinal direction and a contact area between the power rail contact plug and the power rail line are increased, to further help to reduce a resistance of the power rail contact plug and a contact resistance between the power rail line and the power rail contact plug.