Previous work has shown that the employment of a gap drainage impeller in a centrifugal pump can improve the pump's hydraulic performance and cavitation resistance. However, during experiments, an unconventional cavitation phenomenon has been observed in the form of a staggered pair of fixed impeller flow tunnels. For the purpose of understanding the factors involved with this unconventional phenomenon, the present study analyzes the cavitation formation and evolution processes using numerical and experimental methods. A scalable detached eddy simulation (SDES) method was employed to address unsteady turbulent flow. First, the method was validated by comparing the performance data and liquid water velocity distributions obtained by calculation and experiment in the absence of cavitation. Then, numerical simulations of the cavitation flow field were conducted under a flow discharge condition one-half that of the optimum value. Within a particular range of cavitation numbers, the calculated results are found to reproduce the unconventional cavitation phenomenon observed in the experiments. The formation mechanism involves a combination of many factors such as impeller geometry, inflow discharge condition, and cavitation number. As for a certain geometry, the formation and evolution processes can generally be analyzed and explained according to the influence of the attack angle, which is affected by variations in the allocated flow discharge and cavitation volume in each impeller tunnel. The jet flow through the gap between the main and vice blades also contributes to the formation of this phenomenon.