Abstract: In order to solve the problems that the structure of the shielding effectiveness (SE) measurement system of the reverberation chamber (RC) is too complex and inaccurate, a method for measuring the SE of the mutual-coupling RC material based on the boundary deformation technique was put forward. By normalizing the internal power density of the transmitting RC, a modification method for the definition of SE of materials in the RC was proposed. This definition method can be employed to eliminate the influence of the test device and the test material loading on the electric field distribution, so the test accuracy is improved. The performance of the SE test system was studied experimentally, and the influence of the position and direction of the receiving antenna on the SE test results was discussed. The results show that the dynamic range of the developed SE testing system is 60~90 dB, and the standard deviation of the internal electric field is less than 3 dB, which can be used for SE testing. The average SE of the tested composites fluctuates between 28 dB and 34 dB in the frequency range of 1~10 GHz. The maximum standard deviation of the five groups of test data is 1.96 dB, and the coefficient of variation is less than 8.05%, which showed that the test results had good repeatabilty.