Variability phenomenon in active galactic nuclei (AGNs) provides a powerful constrain on their central engine. For a long time the long-term multi-wavelength photometric monitoring plays an important role in obtaining the optical variability of QSOs. However, it just monitors the flux variability in a few points in the wavelength which usually consists of many components including continuum and strong emission line (although the photometric data are accurate). So it is necessary to study the optical spectral variability using spectroscopic monitoring data.