Cancer poses a significant threat to human health, with tumor invasion and metastasis being the leading causes of mortality. Circulating tumor cells (CTCs) and other circulating entities are pivotal in the metastasis process. Consequently, tracking these cells and particles is critical for understanding metastasis. Optical in vivo flow cytometry (IVFC), an innovative laser-based technique, facilitates non-invasive tracking of CTCs and tumor-associated particles, proving invaluable in cancer researches, especially in metastasis studies. This study focuses on the applications of IVFC in tumor metastasis research. We discuss the detection principles of IVFC, highlighting various optical techniques such as fluorescence emission, photoacoustic effects, and computer vision. The categorization of IVFC technologies, including fluorescence in vivo flow cytometry, photoacoustic flow cytometry, and in vivo imaging flow cytometry, is detailed. We review its application in research on different cancers such as liver, prostate, breast cancer, and melanoma. The paper concludes with future prospects for IVFC in studying tumor metastasis.