The Advanced Space-based Solar Observatory (ASO-S) is a mission dedicated to solar physics and is the first Chinese satellite for solar physics study. The ASO-S mission was proposed by Chinese solar community in 2011. The scientific objectives can be summarized as '1M2B'. Here, '1M' stands for magnetic field while '2B' for the two major eruptive phenomena (bursts) on the Sun: solar flares and coronal mass ejections (CMEs). The mission aims at exploring connections among solar magnetic field, solar flares, and CMEs. ASO-S mission has three payloads onboard: the Full-disk solar vector MagnetoGraph (FMG), the Lyman-alpha Solar Telescope (LST), and the solar Hard X-ray Imager (HXI). They are proposed to measure solar magnetic field, to observe CMEs and solar flares. The unique combination of these payloads allows simultaneous observations of vector magnetic field of the full Sun, imaging spectroscopy at high energies of solar flares, formation and evolution of solar flares and CMEs on the disk and in the inner corona. It will not only advance our understanding of the underlying physics of solar eruptions, but also help to improve forecast capability of space weather. ASO-S is formally approved by the Chinese Academy of Sciences (CAS) under the Strategic Priority Research Program on Space Science in June 2017 and scheduled to be launched in early 2022. The nominal mission life is 4 years.