The invention discloses a device suitable for an instant dam break test and an operation method of the device and relates to the field of hydraulics model testing. The device includes a test water groove, a water gate, a transmission system and a triggering system. Before a dam break test starts, a gate simulating a dam body in a door groove is in a static state. When the dam break test starts, aheavy bob falls so as to pull a thin cord, so that a pin is unplugged. A cam is pulled away instantly by a spring. The gate loses support of a support rod and acquires pulling force of a transmissionsystem at the same time, and falls rapidly in an extremely short time. By adopting the device provided by the invention, formation of under-gate hydraulic jump and disturbance of upstream water body due to upward lifting of a gate in a traditional dam break test are avoided, and more accurate study of water flow motion characteristics in a dam body instant break condition is facilitated.