In reinforced concrete (RC) tank walls, a horizontal construction joint exists because of the interruption of concrete casting, resulting in the old and new concrete not forming a reliable connection. The tank walls are prone to experiencing seepage under high hydrostatic pressure. In this paper, the permeability test of concrete with different mix proportion and simulated horizontal construction joints was first conducted. Their seepage behavior was investigated. The test results show that the addition of polypropylene fibers and cementitious capillary crystalline waterproofing materials (CCCW) can improve the impermeability of concrete. The impermeability at the construction joint is much lower than that of single-casting concrete. Moreover, applying CCCW to the water-facing surface of the construction joint can significantly enhance its impermeability. Then, two 1∶2 scaled RC tank wall specimens were tested for local permeability performance of single-casting and double-layer casting considering construction joints. The experimental results show that the RC tank wall with construction joint is prone to generate initial defects and seepage due to inadequate pouring, vibration, and curing of the concrete at the construction joints, making it difficult to achieve the design watertight requirements. The use of modified concrete materials and surface treatment in the relevant part can effectively improve the impermeability of the local construction joint, and further enhance the impermeability of RC tank walls. Finally, the BP neural network was used to establish a nonlinear prediction model between the impermeability of concrete and its material parameters and construction technology.