The immune system is characterized by complexity, diversity, and plasticity. Current research on the composition and dynamic changes of the immune system is limited to its population effects, and cannot fully reflect its heterogeneity. The emergence of single-cell technology has revolutionized our understanding of immunology. Single-cell RNA sequencing (scRNA-seq) is currently the most widely used single-cell technology, capable of analyzing a single cell or molecule to reveal the immune response under physiological or pathological conditions. This article provides a comprehensive review of the general process, development, and application of scRNA-seq in immunology, and discusses its future development in immunology.