The present invention is related to an apparatus and a method for producing fibrous debris of test for a nuclear plant. More particularly, producing fibrous debris of test having the uniform length distribution equal to the fibrous debris collected by a plurality of strainer bags while passing through a strainer in length distribution degree, and conducting a test for evaluating the influence of debris in the core due to a loss of coolant accident based on the produced fibrous debris for testing, thereby ensuring reproducibility of a test by the fibrous debris of test having a uniform length distribution and, thereby producing conservative experimental data.