and X-ray bands (e.g., @Vaughan2004 [@Nardini2016; @Reeves2016; @Gliozzi2017; @Lobban2018]). The [published]{} photometric and spectroscopic monitoring data of Ark 120 date back to 1974. Even earlier (1935-1950) there were historical archival plates collected in the Harvard College Observatory that recorded the photometric images of Ark 120 (@Miller1979). [A recent campaign spectroscopically monitored Ark 120 for about 100 days in 2017 with the 2.3m telescope of Wyoming Infrared Observatory (@Du2018). We carried out further observations between 2015 and 2017 using the 2.4m telescope of Yunnan Observatories.]{} We compile publicly available photometric and spectroscopic data of Ark 120 and obtain a total temporal baseline stretching over four decades. By visual inspection, the long-term variations in [the]{} optical continuum display a wave-like pattern with a possible period of $\sim$20 years. Moreover, the broad H$\beta$ profile of Ark 120 is highly asymmetric and varies strongly with time (@Doroshenko2008 [@Du2018]), a [remarkable]{} analogy to [what was discovered in]{} NGC 5548 (@Li2016 [@Bon2016]).

The paper is organized as follows. Section 2 introduces briefly our new monitoring campaign and then presents our construction of a long-term database for Ark 120. Section 3 studies the long-term optical variability and measures the power spectral density (PSD). In Section 4, we 