A device for evaluating the erosion influence of fine-grained sediments after dam removal is provided. The device comprises a regular rectangular water tank. A water supply device is arranged on one end of the regular rectangular water tank. A dam model is arranged in the regular rectangular water tank behind the water supply device. A hydraulic measuring instrument is arranged in the regular rectangular water tank behind the dam model. A lifting device is arranged under the regular rectangular water tank. A fine-grained sand deposit body is placed in the regular rectangular water tank betweenthe dam model and the water supply device. The device for evaluating the erosion influence of fine-grained sediments after dam removal and a method for calculating an erosion amount can solve the problem in the prior art that the influence of dam removal cannot be evaluated in a laboratory, can simulate an erosion experimental device for fine-grained sand deposited in a reservoir and a method formeasuring the erosion amount, and can obtain sediment erosion experimental data after a dam break in the laboratory.