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Hydrodynamic cavitation is used in various engineering fields, such as water treatment and chemical processing. Cavitation initiation is critical to assessing the cavitation effect in cavitation reactors, so cavitation initiation should be studied to improve cavitation in various fields. This paper investigates the mechanisms and characteristics of cavitation initiation in jet pump cavitation reactors (JPCRs) to intensify the cavitation. A criterion derived for cavitation initiation focuses on the fluctuating pressures caused by the velocity gradients. Experiments show that the criterion predicts low pressure cavitation and strong shear dominant cavitation in a JPCR. Then, the characteristics of these two cavitation mechanisms are investigated for various geometric parameters and operating conditions and a cavitation initiation regime map is developed for JPCR. The results show that cavitation occurs more easily, and the two cavitation mechanisms become increasingly more difficult to distinguish in the JPCR as the inlet pressure increases. The velocity gradient and loss coefficient increase with a decrease in suction pressure, which promotes cavitation. The low pressure cavitation and strong shear dominant cavitation are both more likely when the area ratio is relatively small. The induced pressure spectra further show that the low frequency component dominates during the formation of the low pressure cavitation, while the high frequency component dominates during the formation of the strong shear dominant cavitation. This study clarifies the cavitation mechanisms with special emphasis on the fluctuating pressure and the corresponding characteristics in the JPCR to provide guidance for JPCR applications in industry.