A method called Nakamura’s technique, which uses a horizontal-to-vertical Fourier spectrum ratio (H/V ratio) of microtremor to estimate the dynamic characteristics of a site, was introduced by a Japanese scholar Nakamura in 1989. This technique is widely used, and fruitful achievements in scientific research are presented by this technique. Nowadays, this technique is also used to estimate the structural response characteristics by home and abroad researchers. However, the validity of Nakamura’s technique is still in dispute. The ambient seismic noise measurement was conducted inside a reinforced concrete (RC) building in urban area of Beijing. The calculative analysis on velocity data by Nakamura’s technique is done. The analyses indicate that the natural frequency can be obtained validly by the Nakamura’s technique, but the magnification is not accurate. We must use this technique to estimate structural response characteristics carefully, and we suggest other methods should be used simultaneously to compare.