The invention discloses an experimental water channel device for simulating instantaneous and complete break of a dam. The experimental water channel device for simulating instantaneous and complete break of the dam comprises a water channel, a baffle and a buoyancy twisting device. A support is arranged in the water channel, the baffle is fixed at the bottom of the water channel through a hinge device, the buoyancy twisting device is arranged on the support and comprises bearings, bolts, floating rods and floating balls, the buoyancy twisting device is fixed on the upper portion of the support in the water channel through the bearings, each floating rod and each bearing are fixed together, one end of each floating rod is connected to each bolt through each bearing, and the other end of each floating rod is connected to each floating ball. In the initial phase, the floating balls are vertically downward, and the baffle is perpendicularly fixed in the water channel. When the water level in front of the dam rises, the floating balls rise in a floating mode, twist the floating rods so as to twist the bolts, when the water level rises to a certain degree, the bolts are twisted to exceed the boundary of the baffle, and the baffle is instantaneously pressed down due to pressure of a water body in front of the dam. The experimental water channel device for simulating instantaneous and complete break of the dam avoids errors caused by human factors to experiment engineering, and improves accuracy and repeatability of an instantaneously complete dam break experiment.