Since the 1980s, CCDs had been widely used in astronomy, and the opticalvariability became one of the most effective methods to investigate the astrophysics.The study on active galactic nuclei (AGNs) by optical variability caneffectively help us understand their geometric and physical properties. In thisthesis, we studied central supermassive black holes and broad-line region structuresin two types of AGNs by photometry and reverberation mapping. We also investigated effects of aperture size and seeing on photometric results. Opticalphotometry researches focused on multi-band variability of BL Lac object Mrk 501, and the relationship between the minimum timescale of intra-day variability (IDV) and the mass of black hole. In the reverberation mapping, we mainlystudied time lags between variations of broad emission lines and continua, black hole masses, stratification of broad-line region, and properties of emission linesin narrow-line Seyfert 1 galaxies (NLS1s). We monitored BL Lac object Mrk 501 about 7 years using the 1.02 m and 2.4 m telescopes at Yunnan Observatories. The source was found with extremely weak change. During the 7 years, only one relatively reliable micro-variability was detected on 2010 May 15, and its change amplitude is ～ 0.03 mag. The other observational data can not pass both variability tests (the F-test and the ANOVA test). The host of Mrk 501 is an elliptical galaxy with a very large effective radius and a surface brightness distribution being relatively uniform. So, its photometry can not be done by the common photometric methods for point like source. Based on our observational data, we found that the AGN contribution to the whole galaxy is ～ 13.3 %, and the seeing and aperture can seriously influence the photometric results. Thus, the researches on the optical variability of Mrk 501 need the very careful subtraction of the host galaxy contribution. Also, we studied 9 NLS1s using reverberation mapping method. The time lag, luminosity and full width at half maximum (FWHM) of Hα are measured by fitting and original spectrum, and the mass of the central supermassive black hole is estimated by the time lag and FWHM. Both of mass measurements are consistent with each other. This means the measured results are reliable. Compared to the measurements based on Hbeta reported in other works, we found that there is obvious ionization stratification for two out of these 9 NLS1s. These two sources are obviously different in time lags and FWHMs for Hα and Hβ. The ratio of luminosity is far away from the theoretical expectation L(Hα)/L(Hβ) = 3.1. We analysed the relationship of the black hole mass with the time lag and FWHM. We found that MRM is mainly influenced by the time lag, and is slightly influenced by the FWHM. We analysed the difference of the results obtained by different methods and different emission lines.
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