In order to reveal the electrical characteristics of the switching process of on-load tap-changer in transformer, the structure and function of the vacuum on-load tap-changer and the operating principle of the switching process are studied in this paper. Based on the electromagnetic transient simulation software PSCAD/EMTDC, the corresponding electromagnetic transient simulation model is built according to the action sequence, the electromagnetic transient characteristics such as current and voltage of on-load tap-changer under normal working conditions are studied, and the electromagnetic transient characteristics of vacuum switch under mechanical fault state are simulated, providing references for the design, selection and state evaluation of on-load tap-changer.