The Advanced Space-based Solar Observatory (ASO-S) is the China's first comprehensive solar dedicated satellite, scheduled to be launched around 2022. The Lyman-alpha (Lyα) Solar Telescope (LST) is one of the payloads of the ASO-S, consisting of three scientific instruments and two Guide Telescopes (GTs). The scientific instruments include a Solar Disk Imager (SDI), a White-light Solar Telescope (WST) and a Solar Corona Imager (SCI), which are aimed to observe the wholeevolving process of various eruptions on the Sun, including solar flares and coronal mass ejections, in multiwavelengths with high temporal and spatial resolutions. To achieve this goal, all the three instruments have an observation mode dedicated to the event observations, called the event mode. In this mode, the SCI takes full-frame coronal images at a higher cadence, while SDI and WST take partial frame images around the eruption region at a higher cadence.The time and location information of the eruption events can be effectively obtained by monitoring the relative changes of brightness of each pixel after the median filtering and rebinning the onboard images. The obtained information will provide an important electronic input for the automatic switching between different observation modes of LST.