The scattering inside the computer box is studied. Some results are found through scattering matrix elements. The ensemble-averaged scattering matrix can be obtained by the measured radiation scattering. With the increase of cavity loss, scattering fluctuations are reduced, and insertion phase shift distribution is becoming more and more uniform. Based on the Dyson’s circular ensemble, the scattering behavior within computer box is found to be chaotic. According to RMT, normalized impedance matrix within the microwave chaotic cavity is universal in their statistical description, depending only upon the value of a single dimensionless cavity loss-parameter.