Optical molecular imaging is more and more widely used in the field of biomedical sciences due to its advantageous properties such as non-invasive, real-time and high resolution. As a kind of important optical molecular imaging probe, the near?infrared fluorescent (NIRF) dyes exhibit less tissue absorption and strong tissue penetration, and has been gradually applied to the early diagnosis of tumors. Researchers have developed a number of NIRF dyes with high potential for clinical application by conjugating tumor-targeting ligands, nano-modifications and multimodal NIRF imaging, and have significantly improved the specificity and sensitivity of these NIRF dyes in tumor diagnosis. In this paper we provide a review on the application of NIRF dyes in tumor diagnosis.