Abstract: In order to understand the development mechanism of cloud cavitation and investigate the influence of jet flow on the suction surface of hydrofoil, the unsteady evolution process of cloud cavitation of the original NACA66(mod) hydrofoil and the hydrofoil with jet flow is simulated and compared by using density modified RNG k-\varepsilon turbulence model and Schnerr-Sauer cavitation model. Monitoring lines of the hydrofoil suction surface are adopted and then spatiotemporal evolution of the vapor phase volume fraction, re-entrant jet flow, pressure and pressure gradient near the hydrofoil wall is obtained. The flow field characteristics of the cloud cavitation are analyzed by vortex dynamic. The suppression mechanism of active jet is analyzed. The results show that the first retraction of the attached cavity is caused by the strong local high pressure when the dissociated cavity collapse downstream, and the second retraction is affected by the re-entrant jet. The re-entrant jet zone is limited to higher pressure gradient. High pressure gradient always exists, but the initial occurrence of re-entrant jet in a time period needs the accumulation of time. Jet flow on the suction surface of hydrofoil can reduce cloud cavitation. The suppression mechanism is that injected jet makes the pressure rise near jet holes, making up for the pressure drop caused by cavitation and circumfluence, so as to increase the pressure gradient and enhance the anti-pressure ability. On the other hand, the re-entrant jet strength is decreased. The vortex structure contour of Q criterion shows complex flow structure compared to the vapor phase volume fraction contour. Vortex cores are existed in the front attached cavity and dissociated cavity, while shear may be present in re-entrant jet zone. The shear function of the injected jet suppresses the development of cavitation and re-entrant jet.