The invention discloses a device for simulating influence of fragments on pressure reduction of a fuel assembly and a cold section cleavage simulation method. According to the device, a full-size simulation fuel assembly at a full-scale of 1 to 1 is used; actual working conditions of nuclear power plants can be more really reflected; fiber fragments used by the device are treated by a special method, so that the characteristic parameters of the fiber fragments are closer to the actual conditions of the nuclear power plants after accidents occur; with comparison between the same type of tests at home and abroad, the flow is adjusted and stabilized by a variable-frequency adjustment centrifugal pump; the fluctuation range of the flow is extremely small; when the flow of the loop needs to bechanged, the flow of the loop can be changed just through digital setting; the conditions of cold section cleavage accidents and hot section cleavage accidents can be respectively simulated. Accordingto the device, the fragments can be added into a water tank; in addition, a fragment adding system is additionally arranged in the front of a test section entrance; the possibility of accumulation ofthe fragments in the water tank, the centrifugal pump and the valve can be reduced; the test benefit is improved.