The tip clearance is unavoidable due to the rotation of the impeller during the operation of the hydraulic machinery. The tip leakage vortex and its cavitating flow due to the tip clearance can easily lead to the reduction of key performance parameters such as the pump head and efficiency, and with the enhancement of adverse phenomena such as vibration and noise, which seriously deteriorated the safe, stable and efficient operation of the axial-flow pump, and even caused damage to the pump device. In order to study the effect of tip clearance on the cavitation in a pump unit, slanted axialflow pumps without tip clearance and with three different tip clearances (δ/R=0.0010, 0.0033 and 0.0067) were numerically studied based on SST-CC model and ZGB cavitation model. The study focused more on the correlation between the cavitation and energy performance of the pump. The results showed that the tip clearance mattered the energy performance of the slanted axialflow pump under both free and cavitation conditions, and the energy loss due to this got larger as the tip clearance was increased. The cavitation flow pattern of the case with narrow tip clearance (δ/R=0.001) had little difference from that of the case without tip clearance, and both of them presented as the blade surface cavitation. However, in the cases with the larger tip clearances (δ/R=0.0033 and 0.0067), the sheet cavitation clearly appeared and merged with the blade surface cavitation to form a wedge-shaped cavitation. The cavitation led the phenomenon that water convergence and flow separation on the suction surface, and the interaction between the flow separation and wake flow from the impeller resulted in the passage vortex. This phenomenon was shown to be more pronounced and higher as the tip clearance appeared and increased. Besides, the cavitation and tip leakage flow regions were highly coexisting, indicating that the sheet cavitation was generated by the cavitation bubbles carried by the tip leakage flow. The energy performance of slanted axialflow pump under the cavitation condition was mainly caused by the passage vortex and the tip leakage vortex, and the intensity, scope and energy loss caused by both vortexes was increased with the increase of tip clearance size.