BL Lacertae (BL Lac) objects are the most extreme subclass of active galactic nuclei. One of the distinctive properties of BL Lac is rapid and large amplitude flux variability at all frequencies on different timescales. Owing to the limitations of current observations, direct imaging of the nuclear zones is not possible, rapid variability and correlated variations of the emission at different wavelengths represent the effective investigation tool of the nuclear zones. Such as the physical mechanism of the jet formation, the internal structure of active galactic nuclei, the Origin of high energy gamma-rays, the physical mechanism of a binary system of massive black holes at the center of the nucleus. Through many decades of research, great improvements have been made both in observations and theoretics for active galactic nuclei. However, it is currently unclear what the physical mechanism behind variability of active galactic nuclei. The thesis present the results of the research of long-term optical monitoring of a famous blazar S5 0716+714, such as intra-day variability, quasi-periodic oscillation and color index/spectral index variability. In order to explore the characteristics of multi-color spectral properties of the BL Lac object S5 0716+714 during the outburst, We have monitored the BL Lac object S5 0716+714 in the optical bands with long time spans on intra-day timescales (> 5hr) and high time resolutions(< 6min) during 2012 January and February, the observation lasted for 13 nights. The main results are: the object shows violent flaring activity both in short and intra-day timescales，intra-day variability (IDV) is found on seven nights with at least two bands detected as IDV on a night. The variability amplitude is from 12.81% to 33.22%, The overall magnitude variabilities are △B=1m.24, △V=1m.42, △R=1m.3, △I=1m.23 respectively. There are good inter-bands correlations but not significant time lags for intra-day and short timescales. The variability timescales range from 77.76 minutes to 192.9 minutes. Most of nights show bluer when brighter (BWB) chromatic trend, The BWB trend appears in the short timescales. During the flare the spectral index exhibits a clockwise loop for inter-nights. A shock-in-jet model and the shock wave propagating along a helical path are likely to explain the variability and color index variability. In order to search for the evidence of quasi-periodic oscillation (QPO) in blazars, the data of multi-color optical observations of the BL Lac object S5 0716+714 were performed from 2005 to 2012 have been analysed. For the I band on 2010 March 19, the same QPO ? 50 minutes at the 99% significance level is confirmed by the ZDCF method, Lomb–Scargle method, REDFIT, and fitting sinusoidal curves. The observed QPO is likely to be explained by accretion disk variability.
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