ACT was developed based on the structural marker map and the cell type enrichment algorithm, with the aim of efficiently facilitating the process of cell type annotation. Users are able to explore and annotate cell clusters of interest via convenient and easy-to-use modules implemented in ACT, including the core enrichment tool for annotating cell types, summary tables for enriched results, well-organized hierarchy map of enriched cell types, mapping relationship between canonical markers and differently up-regulated genes, the prevalence of canonical markers and their visualization in integrative multiple organ scRNA-seq expression data of human and mouse.