Abstract: The load of expressway tunnel substations features significant time-varying and seasonal characteristics, so the reactive power compensation design must take both rated and light-load operating conditions into account. Taking a 35kV tunnel substation as an example, this paper analyzes the problems of inductive reactive power under rated conditions and excessive capacitive reactive power caused by long cables under light-load conditions. A mathematical model is established to calculate the capacitive reactive power compensation capacity required by transformers and loads, and evaluate the charging power generated by 30km cables as well as its impacts on voltage and power factor. On this basis, a hybrid reactive power configuration scheme featuring "capacitive compensation as the main part and inductive compensation as the supplement" is proposed, with a comparative analysis of the technical and economic performance between shunt reactors and Static Var Generators (SVG). The results show that shunt capacitors should be configured under rated conditions, while shunt reactors are needed under light-load conditions to suppress voltage rise. This design achieves full-condition reactive power balance, improves power quality and system operation economy, and provides a reference for similar projects.