Emphasis is placed on the performance of a two-stage pump equipped with an eroded impeller. Non-contact reverse engineering technique is utilized to acquire the point data constituting the three-dimensional geometrical model of the eroded impeller. With computational fluid dynamics, both the operation performance of the pump and flow patterns adjacent to the eroded blade surface are obtained. For comparison, the corresponding undamaged impeller is investigated as well. Preferable fidelity of the approach of constructing geometrical model of the damaged impeller is substantiated. The mechanism underlying performance degradation of the pump with the eroded impeller is evidenced. Local flow patterns near the eroded area are featured by explicit irregularity. Furthermore, between the defective and the undamaged pumps, disparity in positions where cavitation occurs is negligible.