To delve into the influence patterns of leakage inlet size and shape on the leak detection by the pseudo-flow field method, we employed numerical modeling of dam leakage to simulate real-world detection scenarios. We investigated the fundamental distribution characteristics of the pseudo-flow field in reservoirs and revealed the distribution patterns of current density near leakage inlets of different sizes and shapes. Empirical formulas were proposed to determine the range of influence and corresponding peak current density of leakage inlets of different sizes under two scenarios, namely, inlet parallel to the dam axis, inlet parallel to the dam surface and perpendicular to the dam axis. It is found that the shape of the inlet can be controlled by adjusting the length-to-width ratio, the current density of the linear leakage inlet is lower than that of the circular one of the same area. Additionally, the influence range of the linear leakage inlet along the short edge is larger than that of the circular one of the same length, whereas that along the long edge is smaller. The study revealed the influence of the size and shape of leakage inlets on the distribution of current density along monitoring lines, providing a basis for the determination of the size and shape of leakage inlets according to the distribution of current density along monitoring lines in practical detection.