Random vibration test is common in mechanics engineering field, where the usually used typical spectrum is to general to truly reflect the vibration environment the products experience, while the methods used to describe the actual vibration based on measured signal, envelope method for example, is mostly qualitative with no statistical indicators to describe its accuracy. With the continuous improvement of random vibration test device, narrowband vibration is easy to achieve, which leads to demands of precisely described random vibration test. Based on statistics theory, the paper elaborates a quantitative method to make a vibration specification spectrum. By testing, merging, modeling and inducting the multi-channel, multi-band and multi-sub-sample data it is realized to quantitatively describe the random vibration environment in a certain confidence level.