Early diagnosis of tumor is a key factor for efficient diagnosis and treatment of cancer. Fluorescence imaging technology, particularly utilizing near-infrared (NIR) light, has shown promising potential in the biomedical field, specifically in the early detection of cancer. NIR light has minimal absorption and scattering in biological tissues, allowing for high-quality imaging with high resolution and signal-to-noise ratio. Realization of high-quality NIR fluorescence imaging relies on excellent fluorescent probes. Upconversion nanoparticles (UCNPs), excited by NIR light, have soared in fluorescence imaging due to their favorable characteristics such as low toxicity, narrow-band emission, tunable emission, long fluorescence lifetime, good stability, and high quantum yield. This article summarizes the basic principles, synthesis methods, and modification/decoration techniques of upconversion fluorescent probes, mainly expounds on several typical imaging modes of rare earth doped upconversion fluorescent probes and their latest research progress in tumor visualization, and prospects the further application research on the integration of diagnosis and treatment.