External water intrusion during low or negative pressure transient events in urban water supply systems may cause pollution and health problems. The volume of intrusion pollutants is one of the significant indicators that can reflect the degree of health risks when a pollution incident occurs. A pilot-scale platform was constructed in this study to simulate intrusion events, which were caused by the sudden valve closure in a laboratorial water distribution system. The simulation aimed to determine the critical factors affecting the intrusion volume during low or negative pressure events and to present an intrusion volume model. Intrusion volumes were measured under different conditions with different flow velocities, internal and external pressures, and orifice diameters. The intrusion volume was considerably affected by the size of leakage points, initial flow velocity, and external pressure at leakage points. It also had a positive correlation with each of the three factors. Theoretical intrusion volumes were calculated with the orifice equation and compared with the measured intrusion volumes. A correction coefficient was introduced to improve the traditional equation of the intrusion flow rate.