With a large distribution of karst all over the world, the research on karst landscapes has gone through a long history, and karst scientists have devoted themselves to it globally. Based on the strong support from UNESCO/IUGS, five International Geoscience Programme (IGCP) ran from 1990 to 2015. IGCP 661 is the sixth karst-related IGCP project, implemented from 2017 to 2021. Ten karst scientists from seven countries lead the international working group to conduct this project. Various issues of karst were detected previously, covering karst formation, carbon cycle, ecological systems and water resources. On this occasion, IGCP 661 paid attention to earth’s critical zone in karst areas, the karst critical zone, combining the lithosphere, hydrosphere atmosphere and biosphere together, which are incompletely understood. With the effort made by global karst scientists and the regular communication, several achievements were achieved in several fields, including basic recognition of karst and critical zones, hydrogeological structure, karst carbon cycles and the atmospheric carbon sink. Besides, with other S&T project support, an initial international monitoring station network was established enhancing the joint research capacity of the working group, also providing a demonstrative construction model for the monitoring station for future work.