The present invention discloses a pressurized test device and method for in-situ mining natural gas hydrates by jets, relating to the field of exploitation of marine natural gas hydrates. The device comprises an injection system, a jet breakup system, an annular pressure system, an axial pressure system, a backpressure system, a vacuum system, a simulation system, a collecting and processing system and a metering system, all of which can operate independently by controlling pipe valves on pipelines. The loading of the confining pressure of the device is independent of the loading of the axial pressure, without interference to each other. Meanwhile, the jet breakup process of natural gas hydrate-containing sediments can be observed in real time by a video camera.