In order to evaluate the shielding ability of materials under the radiation of electromagnetic pulse (EMP), a test method based on GTEM cell is put forward. And the test theory and the components of test system are introduced. The shielding box method is verified to be reasonable with simulation. The shielding effectiveness (SE) of one material is tested with this method, and the test results are compared with the results based on the coaxial method. The results show this test method can be used to test the effect of SE on EMP, and the test results can reflect the true shielding ability of the test material.