As one of the payloads for the Advanced Space-based Solar Observatory (ASO-S) mission, the Lyman-alpha (Ly alpha) Solar Telescope (LST) is aimed at imaging the Sun and the inner corona up to 2.5 R-circle dot (mean solar radius) in both the Ly alpha (121.6 nm) and visible wavebands with high temporo-spatial resolution, mainly targeting solar flares, coronal mass ejections (CMEs) and filaments/prominences. LST observations allow us to trace solar eruptive phenomena from the disk center to the inner corona, to study the relationships between eruptive prominences/filaments, solar flares and CMEs, to explore the dynamical processes and evolution of solar eruptions, to diagnose solar winds, and to derive physical parameters of the solar atmosphere. LST is actually an instrument suite, which consists of a Solar Disk Imager (SDI), a Solar Corona Imager (SCI), aWhite-light Solar Telescope (WST) and two Guide Telescopes (GTs). This is the first paper in a series of LST-related papers. In this paper, we introduce the scientific objectives, present an overview of the LST payload and describe the planned observations. The detailed design and data along with potential diagnostics are described in the second (Paper II) and third (Paper III) papers, respectively, appearing in this issue.