Tumor is a great threat to human health, the majority of tumor patients die from metastatic tumor rather than primary tumor. Circulating tumor cell (CTC) is an important indication of metastatic tumor, and its detection can be used for monitoring tumor metastasis and prognosis evaluation. Based on the difference of light absorption between CTC and blood background, in vivo photoacoustic flow cytometry (PAFC) can realize CTC in vivo detection. Compared with the traditional CTC detection methods, the advantage of PAFC is that it can detect the number of CTC in the circulatory system, and the detection is noninvasive and high sensitive. This review summarizes the methods of the real-time and label-free detection of melanoma CTC and the real-time detection of breast cancer by nanoparticle targeting, and their application in evaluation of tumor metastasis and treatment effect.