| This paper systematically summarized the methods for time-series studies on ocean acidification (OA), and reviewed the advances in long-term tendency and vital controlling factors for OA. The ocean absorbs the atmospheric CO2 substantially and decreases its seawater pH and calcite saturation level (Ω). This phenomenon is known as ocean acidification.Time-series studies on OA mainly focus on the real-time, dynamics, and long-term variations of pH and Ω in diverse marine environments. In time-series studies, pH is usually measured in-situ by sensors attached on mooring buoys, and Ω is calculated by related carbonate parameters. There are 21 active mooring buoys working for the time-series studies on OA in the global ocean, which were located at open ocean, coastal region and coral reef, recording the variation of OA during recent decades. Effected by upwelling, primary production, and terrestrial discharge, pH andΩ in coastal regions possessed significant seasonal and annual variations, which ranged from 7.780 to 8.723, 1.07 to 9.25. However, pH and Ω in open oceans and coral reefs had minor seasonal and annual variations, which varied from 7.890 to 8.238, 7.837 to 8.273, and from 1.93 to 4.19, 2.06 to 5.22, respectively. Considering the multiple effects from anthropogenic activities and climate change, results of timeseries studies on OA suggest the Ω in some coastal regions of Northern Hemisphere has surpass the threshold of some marine organism, such as oyster and mussel, indicating immediate action should be taken to solve the negative effects of OA.