To improve the operational safety of earth rockfill dam, a longitudinal reinforcement is proposed. This rigid structure has triple functions as the seepage controlling, stress resistance and deformation resistance. Based on the classical mechanics, its anti-seepage effect, anti-deformation ability and the capacity to withstand external force is quantitatively analyzed. In addition, the quantitative relationships between the reinforcement thickness and the downstream water level, as well as the material properties of the rockfill dam are derived. For the theoretical derivation, the reinforced body is regarded as a fixed-end beam, loaded by water pressures from upstream and downstream, the active earth pressure from rockfill and the vertical drag force due to the differential settlement between core walls and rockfill. The top displacement together with the rotation angle and the bottom stress of the fixed end are derived from the deformation differential equation and validated through designing reviews. Subsequently, the construction method of buried steel pipe, which is adopted to perform the base grouting and also to meet the strength requirements, is put forward. The proposed method plays an important guiding role in rehabilitating and reinforcing the dangerous earth rockfill dams, as well as the design and construction of new dams.