Abstract: Microwave anechoic chamber is one of the most important test sites in the microwave band, which is widely used in antenna measurements, target scattering parameter measurements and radar imaging. The investment construction of the microwave anechoic chamber is relatively high, and the quiet zone performance of the chamber depends on the characteristics and layout of the absorbing materials laid on its inner wall, while the quiet zone parameter of the chamber is closely related to the measurement accuracy of the devices measured in the chamber. Therefore, acquiring the accurate anechoic chamber quiet zone parameter through measurements not only is an essential step for the site validation, but also the basis for estimating the contribution of the anechoic chamber environment to the measurement results. The method to measuring the quiet zone reflectivity level of the chamber, which is the key parameter of the quiet zone, is discussed in this article, and a quiet zone measuring system based on a three-dimensional scanner with a maximum travel of 4 m is established for medium-to-large microwave chamber covering the frequency range of 1 to 40 GHz. An example of reflectivity level measurement for an anechoic chamber is presented, the measured results are shown, and the uncertainties component which effect the results are discussed, and the total uncertainty for the quiet zone reflectivity level is estimated for the first time. The results show that the expanded uncertainty is 1.72 dB (k=2) at 1.1 GHz for reflectivity level of −33dB.